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Extends WordPress theme.json block style variations with custom properties for external stylesheets.

Overview

WordPress supports defining block style variations in theme.json partials located in the /styles/ directory. However, the native implementation doesn't support linking external CSS files to style variations, either by style_handle or by file: reference.

This plugin adds support for this feature through a custom property in your theme.json variation files.

Usage

Custom Property

Add this custom property to your block style variation JSON files in themes/your-theme/styles/.../*.json:

stylesheet (string)

Path to an external stylesheet or a registered style handle.

• File references: Use "file:./path/to/file.css" to reference a CSS file relative to the JSON file's location
• Style handles: Use a string matching a registered WordPress style handle

Example

File: themes/your-theme/styles/blocks/button/primary.json

{
  "$schema": "https://schemas.wp.org/trunk/theme.json",
  "version": 3,
  "title": "Primary",
  "slug": "primary",
  "blockTypes": ["core/button"],
  "stylesheet": "file:./primary-button.css",
  "styles": {
    "color": {
      "background": "var:preset|color|primary",
      "text": "var:preset|color|white"
    }
  }
}

File: themes/your-theme/styles/blocks/button/primary-button.css

.is-style-primary {
  /* Additional styles requiring media queries or other CSS-only features */
  @media (max-width: 768px) {
    padding: 1em 2em;
  }
}

File Path Resolution

File paths are resolved relative to the JSON file's directory:

• JSON at: themes/your-theme/styles/blocks/button/primary.json
• Reference: "stylesheet": "file:./styles.css"
• Resolves to: themes/your-theme/styles/blocks/button/styles.css

Use ../ to reference parent directories:

• Reference: "stylesheet": "file:../../shared/button-base.css"
• Resolves to: themes/your-theme/styles/shared/button-base.css

Using Pre-registered Handles

If you've already registered a style handle via wp_register_style(), reference it directly:

{
  "stylesheet": "my-custom-handle"
}

Versioning stylesheets

WordPress normally sets block asset versions based on the version string in block.json. Since the schema for block style variation JSON partials does not include a version number, you can manually specify a version string in your stylesheet reference:

"stylesheet": "file:/variation-styles.css?ver=1.0.2"

If you omit the ver parameter, a version string will be computed by hashing the stylesheet. This requires an inefficient full-file read operation on every registration call, so this auto-versioning should only be used as a fallback convenience while actively developing styles.

How It Works

1. WordPress core reads theme.json partials from /styles/ and registers block style variations
2. This plugin scans the same directory for JSON files containing a stylesheet property
3. For file: references:
   • The path is resolved relative to the JSON file's location
   • A unique style handle is generated based on the file path
   • The stylesheet is registered with WordPress (including RTL support and path data for inlining)
4. Stylesheets are enqueued via wp_enqueue_block_style():
   • If block assets load on-demand: Enqueued when blocks with the variation class render
   • Otherwise: Enqueued immediately
5. WordPress handles merging the styles with theme.json definitions and potential inlining

Compatibility

• WordPress: 6.6+
• PHP: 8.0+

This plugin extends core WordPress functionality and follows the same file structure conventions as WordPress core's theme.json parser.

API Reference

Extended_Block_Variations\register_variation_stylesheets(): void

Enqueues stylesheets for block style variations that define a custom stylesheet property. Handles both immediate and on-demand loading based on theme configuration.

Performance Optimizations

The plugin integrates with WordPress's block asset loading strategies:

• On-demand loading: When enabled, stylesheets only load when blocks with the variation are actually rendered
• Automatic inlining: Small stylesheets (under 40KB by default) may be inlined automatically
• RTL support: Automatically detects and loads RTL versions of stylesheets when available
• File versioning: Uses file modification time for cache busting

Integration with Theme

WordPress core automatically registers block style variations from theme.json partials in the /styles/ directory. This plugin enhances that process by:

1. Detecting variations with custom stylesheet properties
2. Resolving and registering the referenced stylesheets
3. Enqueueing them alongside the core-registered block styles

The plugin hooks into after_setup_theme, running before block registration to ensure stylesheets are ready when blocks render. No additional configuration is required - simply add the stylesheet property to your variation JSON files.

Setting Default Block Styles

To configure default block styling, use theme.json to set styles at the block level rather than the variation level. This is the WordPress-native approach and ensures proper inheritance and overrides.