No – BUT
I recently had the pleasure of upgrading a very legacy WordPress site. Not my build, definitely showing its age, but it had one useful thing: the CSS was built with SASS, and the client only wanted a brand colour change.
With legacy WordPress you usually have three choices:
- Rebuild and migrate
- Upgrade what you can
- Ignore it and hope it behaves
A full rebuild isn’t always an option, especially when there’s a large amount of older content. Migrating everything into blocks can be time-consuming, and clients still expect to edit their existing pages. Telling a client “you can’t edit pages anymore” is rarely an acceptable answer. Older pages also tend to contain shortcodes, custom fields or builder markup, which don’t always convert cleanly into blocks and can easily break layouts.
So what can you do?
Change 1:
Enable Gutenberg for posts, but leave pages on the classic editor.
Posts are usually easier to modernise because they’re edited less often. Pages can stay untouched and continue rendering as they always have. If the Classic Editor plugin is installed, this happens seamlessly and nothing changes on the front-end.
Change 2:
Add a theme.json file to the legacy theme.
This gives you a single source of truth for colours, fonts and block settings. WordPress automatically turns these into global CSS variables that work in both the editor and the front-end. For example, if you define a primary colour in theme.json, you can reference it in your SASS with var(–wp–preset–color–primary) and never recompile when the colour changes.
Once theme.json exists, the block editor also gains proper design controls: branded colour pickers, typography options, spacing settings and block defaults. These variables are applied anywhere blocks are used in posts or pages, keeping brand styling consistent even in a legacy theme.
No, this doesn’t magically convert the theme into Full Site Editing & Templating , but it modernises what you have. It reduces tech debt, keeps branding and block styling consistent, and brings an old theme closer to current WordPress standards without a rebuild.
It’s not perfect, but it’s a practical upgrade when rebuilding isn’t on the table.
