Why it shows up in your week
Header, footer, and page templates used to live only in PHP that marketing could not touch. A block theme opens those pieces in the Site Editor. That is power and risk in the same login.
In plain language
theme.json holds colour palettes, type scales, spacing scales, and default block styles. Components should consume those tokens instead of one-off hex codes in every pattern.
Template parts (header, footer, sometimes sidebars) become reusable block assemblies. Change the footer once when the legal line updates.
Full Site Editing is the product name for editing those templates in the admin. Not every "modern" theme is a true block theme. Check for theme.json and HTML templates under the theme folder.
Classic themes remain valid when they are fast, maintained, and paired with a disciplined block content model. Switching mid-flight without a design system creates chaos.
Hybrid approaches exist (classic theme plus block templates). Pick one operating model and document it so the next developer does not guess.
Facts worth keeping
- Also called
- Full Site Editing (FSE) themes
- Key file
- theme.json for global styles and settings
- Templates
- HTML template files editable in Site Editor
- Classic alternative
- PHP-centric themes with optional block content
- Editor skill needed
- Patterns, template parts, and global styles, not only page content
Not the same as
- A classic theme with Gutenberg contentPosts may use blocks while header and footer stay in PHP. That is not a full block theme.
- A page builder themeMany commercial themes assume Elementor or similar. That is a different architecture and vendor dependency.
- No-code foreverCustom blocks, performance work, integrations, and accessibility still need engineering judgment.
Where it bites
Block themes hurt when a classic theme is half-migrated, or when every section is a one-off block soup with no patterns.
Teams then lose design consistency and Core Web Vitals in the same sprint, while three people edit the global header in incompatible ways.
What to check
- Is the site on a true block theme (theme.json + HTML templates) or a classic theme with blocks only in post content?
- Are colours, type, and spacing defined once in theme.json?
- Can editors change header or footer without a developer deploy, and should they?
- Are patterns mapped to the design system used in Figma?
- Who approves global template changes before they hit production?
Common questions
What is a WordPress block theme?
A block theme uses HTML templates, template parts, and theme.json so Site Editor and blocks control site chrome and content, not only post bodies.
How is a block theme different from a classic theme?
Classic themes rely mainly on PHP templates. Block themes expose templates and global styles as block data editable in Site Editor.
Should every site switch to a block theme?
Not automatically. Stable classic themes can stay if they are fast and maintainable. New builds and major relaunches often justify a block theme when editors need broader layout control.
Does a block theme hurt performance?
Poorly built patterns and heavy block CSS can. A disciplined theme.json and lean patterns usually perform as well as a clean classic theme.
What should you check first?
Confirm theme type, global style ownership, pattern library quality, and a release path for template changes.