WordPress Block Themes: Complete Guide

The introduction of block themes marks a revolutionary transformation in WordPress theme development, fundamentally changing how developers build and users customize their websites. This comprehensive guide explores the architecture, implementation, and best practices for modern block-based theme development.

Understanding Block Theme Architecture

Block themes represent a paradigm shift from traditional PHP-templated themes to a fully HTML-based template system. Unlike classic themes that rely on PHP functions like get_header() and the_loop(), block themes utilize structured HTML templates that the Site Editor can parse and modify visually. This architectural change enables true Full Site Editing (FSE), where every element—from headers to footers, sidebars to content areas—becomes editable through the block interface.

The foundation of block themes rests on three pillars: HTML template files stored in the /templates directory, reusable template parts in /parts, and the powerful theme.json configuration file that controls global settings and styles. This structure eliminates the need for traditional functions.php bloat while providing more granular control over design systems.

Essential File Structure and Components

A minimal block theme requires specific files and directories. The style.css file remains necessary for theme metadata, but its role in styling has shifted dramatically. The theme.json file now handles most styling concerns through a structured JSON schema that defines typography, colors, spacing, and layout options. Templates must be stored as HTML files in the /templates directory, with required templates including index.html at minimum.

Template parts live in the /parts directory and function as reusable components that can be inserted into multiple templates. Common template parts include header.html, footer.html, and sidebar.html. These parts can be edited independently and changes propagate across all templates that reference them, ensuring design consistency.

The Power of theme.json

The theme.json file operates as the central nervous system of your block theme. Through its schema, you define global presets for colors, gradients, duotone filters, font families, and font sizes. The settings section controls which features are available in the block editor—you can disable custom colors, font sizes, or spacing controls as needed to maintain design integrity.

Styles defined in theme.json cascade through your theme using CSS custom properties, creating a consistent design system. You can define styles at the global level, for specific blocks, or for custom block style variations. This approach generates optimized CSS automatically while maintaining perfect alignment with your design tokens.

Template Hierarchy in Block Themes

Block themes follow WordPress template hierarchy but with HTML extensions instead of PHP. The system looks for front-page.html for the homepage, single.html for single posts, page.html for pages, and archive.html for archives. You can create highly specific templates like single-product.html for WooCommerce products or page-about.html for a specific page slug.

Template parts can be included using the Template Part block with syntax like <!-- wp:template-part {"slug":"header"} /-->. Query loops for displaying posts utilize the Query Loop block with nested Post Title, Post Content, and Post Featured Image blocks, replacing traditional PHP loops entirely.

Development Workflow and Tools

Developing block themes requires a shift in workflow. Instead of writing PHP code, you’ll work primarily with the Site Editor to create templates, then export the HTML and refine it in your code editor. The Create Block Theme plugin provides invaluable tools for exporting customizations, creating variations, and managing theme.json schema.

Testing block themes requires WordPress 6.0 or higher for full feature support. Use the Theme Check plugin to validate your theme meets WordPress.org requirements if you plan to distribute it. Browser DevTools become essential for inspecting the CSS custom properties generated by theme.json and debugging style inheritance.

Advanced Techniques and Customization

Block themes support advanced features like multiple style variations through the /styles directory. Each JSON file in this folder creates a complete style variation users can switch between, enabling dark modes, alternate color schemes, or brand variations within a single theme.

Custom block patterns stored in /patterns as PHP files let you ship pre-designed block layouts. These patterns can include complex arrangements of blocks that users insert with one click, dramatically improving the content creation experience. Patterns support categories and can be locked to prevent accidental modification of specific elements.

Performance and Production Considerations

Block themes generally perform better than classic themes due to reduced PHP overhead and optimized CSS generation. The Site Editor loads styles only for blocks in use, reducing payload size. However, you must still optimize images, leverage browser caching, and minimize JavaScript.

For production deployments, consider using a child theme for client customizations to preserve update paths. Document your theme.json settings thoroughly, as the file can become complex in large projects. Version control your templates carefully, as the Site Editor can overwrite template files if users make changes through the interface.

The future of WordPress clearly lies with block themes. By mastering this architecture now, you position yourself at the forefront of WordPress development, ready to build modern, maintainable, and user-friendly websites that leverage the full power of the block editor ecosystem.