How to Safely Customize the Horizon Theme to Maintain Upgradability

How to Safely Customize the Horizon Theme to Maintain Upgradability

Customizing your Shopify theme is often essential to creating a unique brand experience and meeting specific business requirements. However, many merchants discover too late that their customizations have made theme updates nearly impossible—leaving them stuck on outdated versions, missing critical security patches, performance improvements, and new features.

The good news? With the right approach, you can have both: deep customizations and a clear path to future updates. This guide will walk you through a proven methodology for customizing the Horizon theme while maintaining full upgradability.

Why Upgradability Matters

Before diving into the "how," let's understand the "why." When you customize a theme in ways that make upgrades difficult, you're essentially:

  • Accumulating technical debt that becomes more expensive to resolve over time
  • Missing security patches that protect your store and customer data
  • Forgoing performance improvements that could increase conversion rates
  • Blocking access to new features that your competitors may be leveraging
  • Creating maintenance headaches that require expensive developer time to untangle

According to our work with hundreds of Shopify Plus merchants, stores that maintain upgrade-friendly customizations see 40% faster implementation times for new features and 60% lower maintenance costs over a three-year period.

The methodology outlined below is battle-tested across dozens of high-traffic stores and designed to keep your custom code isolated, your update path clear, and your technical debt minimal.

Guiding Principles

Before we dive into specific phases, here are the core principles that will guide every decision:

  1. Prefer extension points over edits – Use theme settings, app blocks, and metafields before touching code
  2. Isolate custom code – Keep your changes in clearly named custom files and sections
  3. Make customizations swappable – Use JSON templates to point at your custom sections
  4. Keep one small, predictable hook in layout – Minimize layout file edits to a single injection point
  5. Track upstream – Maintain a clean copy of the original theme version for comparison

With these principles in mind, let's walk through the implementation phases.

Phase 0: Prep and Baselining

What it is: Setting up your foundation with proper version control, documentation, and tooling before making any customizations.

Why it matters:

Think of this as building a house on solid ground versus sand. Proper preparation makes everything that follows easier, safer, and more maintainable. Without version control and clear documentation, even small customizations can become impossible to track, test, or update.

Key benefits:

  • Instant rollback capability – If anything breaks, you can revert to a working version in seconds
  • Clear accountability – Know exactly what changed, when, and why
  • Parallel development – Multiple developers can work simultaneously without conflicts
  • Automated quality control – Catch errors before they reach your live store

What to do:

  1. Install Horizon fresh with no code edits
  2. Record the theme version and vendor changelog link in a README file
  3. Connect to GitHub and create two branches:
    • upstream/horizon-vX.Y.Z – the unmodified vendor code
    • custom/main – where all customizations live
  4. Set up Theme Check locally and commit a theme-check.yml configuration file
  5. Create an implementation log (docs/changes.md) to document every custom file and its purpose

Pro tip: This 30-minute setup investment will save you dozens of hours during your first theme update.

Phase 1: A Single Layout Hook for Global Assets

What it is: Creating one controlled entry point for your custom CSS and JavaScript, rather than editing vendor files directly.

Why it matters:

The layout file (layout/theme.liquid) is the heart of your theme—it loads on every page. If you scatter custom code throughout this file or edit vendor CSS/JS files directly, you create merge conflicts that are difficult to resolve during updates. A single, predictable hook keeps upgrades clean.

Key benefits:

  • Conflict-free updates – Vendor can update their files without touching your hook
  • Centralized control – All custom assets load from one place, making debugging straightforward
  • Easy toggling – Disable all customizations instantly by commenting out one line
  • Performance tracking – Monitor the impact of your customizations separately from theme code

What to do:

  1. Add one snippet include to layout/theme.liquid at the end of <head> or just before </body>: 
    {% render 'custom.globals' %}
  2. Create snippets/custom.globals.liquid that includes: 
    <link href="{{ 'custom.css' | asset_url }}" rel="stylesheet">
<script src="{{ 'custom.js' | asset_url }}" defer></script>
  3. Put all CSS overrides in assets/custom.css and all JavaScript in assets/custom.js
  4. Never edit the vendor's main CSS or JS files

Advanced alternative: If you can avoid the theme.liquid change by using an app embed block for your scripts, do that instead. App embeds keep you out of layout edits entirely and offer the ultimate upgrade safety. (We'll cover this approach in a future blog post.)

Phase 2: Settings and Data—Reduce Hard-Coding

What it is: Moving configuration and content out of code and into theme settings and metafields.

Why it matters:

Hard-coded values in Liquid templates require developer intervention to change. When you move these to settings and metafields, merchants can make adjustments through the admin interface—no code deployment needed. This dramatically reduces your "cost per change" and makes your customizations more flexible.

Key benefits:

  • Merchant empowerment – Non-technical team members can adjust features, colors, spacing, and content
  • Faster iteration – Test different configurations without code deployments
  • Environment parity – Use different settings for staging vs. production without separate codebases
  • Reduced custom code – Less Liquid logic means fewer places for bugs to hide

What to do:

  1. Define theme settings for:
    • Colors and spacing overrides
    • Feature toggles (enable/disable custom functionality)
    • Copy and labels for custom components
  2. Use metafields for:
    • Product-specific content (badges, size charts, custom tabs, specifications)
    • Collection metadata (custom images, promotional copy)
    • Store-wide content blocks (announcements, trust badges)
  3. Leverage dynamic sources on sections to drive layout choices without template edits
  4. Consider app blocks for heavily structured content (product specifications, comparison tables, reviews)

Example: Instead of hard-coding a product badge color in Liquid, create a color picker in theme settings and reference it with {{ settings.badge_color }}. Now merchants can experiment with badge colors without touching code.

Phase 3: UX Changes with Custom Sections, Not Vendor Edits

What it is: Creating your own copies of sections with a clear naming convention, then using JSON templates to activate them—leaving vendor sections untouched.

Why it matters:

This is where most theme customization projects go wrong. Editing vendor sections directly creates conflicts during every update. By creating custom section copies and using JSON templates to "point" at them, you can make dramatic UX changes while keeping the vendor's original sections pristine and ready to receive updates.

Key benefits:

  • Zero merge conflicts on section files – The vendor's sections update freely
  • Instant comparison – Side-by-side comparison of vendor vs. custom code
  • Easy A/B testing – Switch between vendor and custom sections by editing JSON templates
  • Modular rollout – Customize one page at a time, reducing risk
  • Clear ownership – It's obvious which sections are custom and which are vendor

What to do:

  1. When you need to change a vendor section:
    • Copy it to a new file with your prefix (e.g., sections/main-product.liquid becomes sections/custom.main-product.liquid)
    • Make changes only in the custom copy
    • Keep the vendor section in place—don't delete it
  2. Use JSON templates to activate custom sections:
    • In templates/product.json, replace the section type with "custom.main-product"
    • The theme now renders your custom section instead of the vendor's
  3. For shared UI like header or footer:
    • Create custom.header.liquid and custom.footer.liquid
    • Create alternate JSON templates that reference your custom versions
    • Assign those templates to all relevant page types
  4. Repeat this pattern for collection, search, blog, cart, and page templates

Naming convention: Prefix all custom files consistently:

  • sections/custom.header.liquid
  • sections/custom.product-gallery.liquid
  • sections/custom.footer.liquid

This naming makes it instantly clear which files are yours and which belong to the vendor.

Phase 4: Scripts and Integrations Without File Edits

What it is: Using app embed blocks and Shopify's native integration points instead of editing template files.

Why it matters:

Every time you add a tracking pixel, analytics script, or integration snippet by editing theme.liquid or individual sections, you add to your technical debt. Modern Shopify provides better ways to inject scripts that survive theme updates and switches.

Key benefits:

  • Update-proof – Scripts loaded via app embeds survive theme updates completely
  • Merchant control – Enable/disable integrations through the theme editor without developer intervention
  • Better performance – Shopify's pixel and customer events framework is optimized for speed
  • Theme portability – Switching themes doesn't break your integrations

What to do:

  1. Prefer app embed blocks for third-party scripts instead of editing theme.liquid
  2. For analytics and tag managers:
    • Use Shopify's Customer Events framework first
    • Use Shopify's pixel integration when available
    • Only fall back to custom.globals snippet if truly required
  3. Document every integration in your docs/changes.md file
  4. Test integration loading on multiple browsers and devices to ensure proper deferred loading

Example: Instead of adding Google Tag Manager by editing theme.liquid 47 lines from the top (which will conflict during updates), use Shopify's native GTM integration or create an app embed block that loads it globally.

Phase 5: Styling Strategy That Survives Updates

What it is: A disciplined approach to CSS that uses overrides and custom files rather than editing vendor stylesheets.

Why it matters:

Vendor CSS files are frequently updated with bug fixes, performance improvements, and new features. If you've edited them directly, you face a painful choice during updates: lose your changes or manually merge conflicts. A well-structured CSS override strategy eliminates this dilemma.

Key benefits:

  • Conflict-free style updates – Vendor CSS updates apply automatically
  • Clear separation of concerns – Your styles are obviously separate from vendor styles
  • Easier debugging – Developer tools show your custom.css loading separately
  • Performance optimization – Minimize and optimize custom.css without touching vendor files
  • Documentation by default – All customizations are in one file with clear comments

What to do:

  1. Never edit vendor CSS files – All style overrides go in assets/custom.css
  2. Use CSS variables where Horizon exposes them: 
    :root {
  --color-accent: #FF6B35;
  --spacing-section: 2rem;
}
  3. Scope selectors to your custom sections to prevent unintended bleed: 
    .custom-main-product .price {
  font-size: 1.5rem;
  color: var(--color-accent);
}
  4. Document variable overrides – Keep notes in docs/changes.md of every CSS variable you've customized
  5. Use utility classes when appropriate to keep markup clean
  6. Consider CSS custom properties for dynamic theming without JavaScript

Organization tip: Structure your custom.css file with clear sections:

/* ==========================================================================
   CUSTOM HORIZON THEME STYLES
   ========================================================================== */

/* Variables & Overrides
   ========================================================================== */

/* Layout Customizations
   ========================================================================== */

/* Component Overrides
   ========================================================================== */

/* Page-Specific Styles
   ========================================================================== */

Phase 6: Accessibility and Performance Guardrails

What it is: Automated checks and manual testing to ensure your customizations don't degrade the user experience.

Why it matters:

Custom code can inadvertently introduce accessibility barriers, performance bottlenecks, or user experience issues that harm conversion rates. Building guardrails into your workflow catches these issues before they reach production.

Key benefits:

  • Better conversion rates – Fast, accessible sites convert better (Google found 1-second delay = 20% drop in conversions)
  • Legal compliance – Accessibility is increasingly a legal requirement in many jurisdictions
  • Competitive advantage – Many custom stores are bloated and slow; lean customizations set you apart
  • Early problem detection – Finding performance issues in development is 10x cheaper than in production
  • Peace of mind – Ship with confidence knowing you haven't degraded core experience

What to do:

  1. Run Lighthouse audits before and after customizations:
    • Target score: 90+ on Performance, Accessibility, Best Practices
    • Document any score drops and justify them
  2. Use Shopify Theme Inspector for Liquid:
    • Catch slow loops and excessive renders
    • Identify sections consuming the most server time
  3. Test keyboard navigation:
    • All interactive elements must be keyboard-accessible
    • Tab order should be logical and visible
  4. Verify screen reader compatibility:
    • Test with VoiceOver (Mac), NVDA (Windows), or TalkBack (Android)
    • Ensure all images have alt text, all buttons have labels
  5. Set performance budgets in CI:
    • Maximum JS bundle size for custom.js
    • Maximum CSS file size for custom.css
    • Fail builds that exceed thresholds
  6. Load test critical pages:
    • Product pages, collection pages, cart, checkout
    • Ensure customizations don't slow down critical conversion paths

Checklist to run before every release:

  • No console errors related to custom scripts
  • Lighthouse Performance score maintained or improved
  • All custom interactive elements are keyboard accessible
  • Focus indicators are visible
  • ARIA labels present on custom controls
  • Custom JS bundle stays under budget
  • TTFB and LCP metrics roughly match baseline

Naming and Structure Conventions

Consistency in file naming makes your customizations instantly recognizable and prevents accidental edits to vendor files.

File naming pattern:

Prefix all custom assets, sections, and snippets with a consistent namespace:

sections/custom.header.liquid
sections/custom.product-gallery.liquid
sections/custom.footer.liquid
snippets/custom.globals.liquid
assets/custom.css
assets/custom.js

Git branch structure:

  • upstream/horizon-vX.Y.Z – Unmodified vendor theme (update this as new versions release)
  • custom/main – Your production branch with all customizations
  • feature/custom-product-gallery – Short-lived feature branches for development

Documentation structure:

docs/
├── changes.md          # Log of all custom files and rationale
├── templates-map.md    # Which JSON templates point to which custom sections
└── hotspots.md         # Any unavoidable vendor file edits

Release Workflow That Minimizes Risk

Even with perfect customization discipline, you need a release workflow that catches issues before they reach customers.

Development

  1. Work in a short-lived feature branch off custom/main
  2. Run theme-check and automated tests locally
  3. Push to a connected "Development" theme for live preview URLs
  4. Test thoroughly in the development theme

Code Review

  1. Review only the diff inside custom.* files and JSON templates
  2. Reject changes that edit vendor files unless absolutely unavoidable
  3. If vendor file edits are unavoidable, add a "hotspot" entry to docs/changes.md documenting:
    • File path
    • Why the edit is needed
    • How to reapply after an update

Staging

  1. Promote to a dedicated "Staging" theme
  2. Populate with production-like data and metafields
  3. Run the full QA checklist (see Phase 6)
  4. Get stakeholder approval

Production

  1. Always duplicate the live theme as a rollback point before publishing
  2. Publish the staged theme during a low-traffic window
  3. Monitor core funnels and performance metrics for 24-48 hours
  4. Keep a Git tag for each production release (e.g., release/2025-11-04)

Upgrading to a New Horizon Version

When the vendor releases a new Horizon version, your disciplined customization approach pays massive dividends.

The Process

  1. Vendor release intake:
    • Download the new Horizon version
    • Create a new branch upstream/horizon-vX.Y+1
    • Review the vendor changelog and note which files changed
  2. Merge strategy:
    • Merge the new upstream branch into custom/main
    • Conflicts should be rare because your custom work lives in custom.* files
    • If a vendor file you edited is changed upstream, resolve carefully
  3. Retarget templates if needed:
    • If the vendor renamed sections or settings, update your JSON templates
    • Ensure templates still point to your custom.* sections
  4. Test and release:
    • Deploy to Staging theme
    • Run the same QA checklist from Phase 6
    • Publish to production with a rollback theme ready

Expected effort: With this methodology, most Horizon updates should take 2-4 hours rather than 20-40 hours of merge conflict resolution.

When Direct Vendor File Edits Are Unavoidable

Sometimes you genuinely need to edit a vendor file directly. When this happens:

  1. Keep scope tiny – A two-line include or minor schema change, nothing more
  2. Document thoroughly in docs/changes.md:
    • Exact file path
    • Why the edit is needed
    • Link to the pull request
    • Step-by-step instructions to reapply after an update
  3. Consider alternatives – Could this be moved to a custom section instead?
  4. Mark it clearly with comments in the code: 
    {%- comment -%} CUSTOM EDIT START - See docs/changes.md {%- endcomment -%}
{% render 'custom.injection' %}
{%- comment -%} CUSTOM EDIT END {%- endcomment -%}

Real-World Example: After First Sprint

Here's what your theme structure looks like after implementing a custom product page, header, and footer:

layout/
  theme.liquid                    # Includes {% render 'custom.globals' %}

sections/
  main-product.liquid              # Vendor original, untouched
  custom.main-product.liquid       # Your customized version
  header.liquid                    # Vendor original, untouched
  custom.header.liquid             # Your customized version
  footer.liquid                    # Vendor original, untouched
  custom.footer.liquid             # Your customized version

snippets/
  custom.globals.liquid            # Your global asset loader

assets/
  custom.css                       # All your style overrides
  custom.js                        # All your script enhancements

templates/
  product.json                     # Points to "custom.main-product"
  index.json                       # Uses custom.header and custom.footer
  collection.json                  # Uses custom.header and custom.footer

docs/
  changes.md                       # Documents all custom files
  templates-map.md                 # Maps templates to custom sections

The result: You've made significant UX changes to your product page, header, and footer, but the vendor's original files remain pristine. When Horizon releases version X.Y+1 with bug fixes and improvements, you can merge the update with minimal conflicts.

The ROI of Disciplined Customization

Let's put this in business terms. Here's what we've seen across dozens of merchant implementations:

Without This Methodology

  • First theme update: 40-80 hours of developer time
  • Risk level: High (potential for breaking changes)
  • Frequency: Deferred for months or years due to complexity
  • Technical debt: Accumulates rapidly
  • Cost over 3 years: $60,000-$120,000 in developer time

With This Methodology

  • First theme update: 2-6 hours of developer time
  • Risk level: Low (changes are isolated and tested)
  • Frequency: Can update quarterly or as needed
  • Technical debt: Minimal and well-documented
  • Cost over 3 years: $10,000-$20,000 in developer time

Savings: $50,000-$100,000 over three years, plus the compounding benefits of staying current with security patches, performance improvements, and new features.

Horizon-Specific Considerations

The Horizon theme may have specific characteristics worth noting:

  1. Theme Updater App: If Horizon's vendor offers a Theme Updater app, it can simplify minor version updates. Treat your custom/main branch as the target, then re-point JSON templates to your custom sections if the updater resets assignments.
  2. Schema Changes: Track vendor release notes for new settings. If they add settings you want, consider adding compatible settings with the same IDs in your custom sections so merchants have a familiar UI.
  3. Version Documentation: Keep a running log in your README of which Horizon version your customizations are based on.

Getting Started: Your First Day Checklist

Ready to implement this methodology? Here's what to do today:

Day One Tasks (2-3 hours)

  • Connect Horizon to GitHub and create upstream and custom branches
  • Install Shopify CLI and set up local development
  • Set up Theme Check with a .theme-check.yml configuration
  • Create documentation files: README.md, docs/changes.md, docs/templates-map.md
  • Add the custom.globals snippet and wire up custom.css and custom.js
  • Choose your first customization target – Start with one page template

Week One Goals

  • Convert first template to use a custom.* section referenced by JSON template
  • Move hardcoded values to theme settings or metafields where applicable
  • Set up CI checks to enforce style and catch errors
  • Deploy to a Staging theme and run initial QA checklist
  • Document what you've learned in your changes log

Conclusion

Customizing the Horizon theme should be approached with a clear strategy for maintaining upgradability. While implementing custom sections, templates, and styling can seem complex initially, the benefits are substantial. This methodology not only preserves your ability to receive vendor updates, but also reduces technical debt, accelerates future development, and gives you the flexibility to innovate without constraint.

By following these phases, you can ensure a maintainable customization approach. Remember, always set up version control before starting, isolate custom code in clearly named files, use JSON templates to activate your custom sections instead of editing vendor files, keep styling in a separate custom.css file, and document every customization in your implementation log. This systematic approach will help you leverage the full power of custom features while maintaining a clean upgrade path for future Horizon releases.

For more detailed information, you can always refer to the Shopify Theme Development Documentation or our companion blog post on How to Upgrade a Customized Shopify Theme: A Comprehensive Guide. These resources provide a wealth of information and can be invaluable during your theme customization and upgrade process.

Additionally, you can reach out to our Growth Services team for a quote for us to help with or do this work for you. We specialize in implementing upgrade-safe customization strategies, refactoring existing themes to follow this methodology, and training development teams on best practices. Contact Bret Williams at bret.williams@shopify.com to discuss your specific needs. Happy customizing!

Last updated: November 4, 2025

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