Mary Rose Elbambo

BlockGlow adds a “BlockGlow — Custom CSS” panel to the inspector of every block. Use it to scope custom CSS to a single block instance. The plugin stores CSS in the uploads folder (under uploads/blockglow/) and enqueues the generated CSS file when rendering on the front end. If the uploads directory isn’t writable or file write fails, BlockGlow will fall back to outputting inline styles.
Key features:
<ul>
<li>Adds two block attributes to every block: <code>blockglowCustomCSS</code> and <code>blockglowId</code>.</li>
<li>Inspector panel with a CodeMirror-backed code editor (if WordPress provides it) or a fallback textarea supporting Tab and Shift+Tab outdent.</li>
<li>Live preview inside the block editor: custom CSS is scoped to the block in the editor so you can see the effect while editing.</li>
<li>CSS files are saved to <code>wp-content/uploads/blockglow/blockglow-<id>.css</code> and enqueued on the front end as a linked stylesheet; files are removed automatically when you clear the Custom CSS field.</li>
<li>Graceful fallback to inline styles when file writing isn’t possible.</li>
</ul>
How scoping works:
BlockGlow creates a unique <code>blockglowId</code> for each block instance and scopes your CSS with a wrapper selector like:
<pre><code>[data-blockglow-id="bg-xxxxxxx"] { /* your declarations */ }
</code></pre>
For convenience you can use <code>&</code> as the wrapper shorthand in the editor. Example:
<pre><code>& .my-class { color: red; }
</code></pre>
The plugin will replace <code>&</code> with the wrapper selector for both editor preview and front-end output.
<h3>Support</h3>
If you find issues or want improvements (for example: limiting who can edit per-block CSS, collecting styles into a single head include, or adding an admin UI to manage generated CSS files), open an issue in the repository or reply here and I will assist.

BlockGlow — Per-block Custom CSS

Adds a "Custom CSS" inspector panel to every Gutenberg block and saves per-block CSS files to uploads for front-end enqueuing.

Daniele De Santis

WP Add Custom CSS allows you to add custom CSS to the whole website and to individual posts, pages and custom post types (such as Woocommerce products). 
The CSS rules applied to the whole website will override the default stylesheets of your theme and plugins, while the CSS rules applied to specific pages, posts or custom post types will override the main stylesheet too.
The plugin works with the most popular builders, such as Elementor, Gutenberg and the Classic Editor.
You can edit the main stylesheet from the the “Add custom CSS” settings page. 
The plugin also creates a new “Custom CSS” box in the editing area to add custom CSS to specific posts, pages and custom post types.
Select the preferred CSS output method from the plugin’s settings page to ensure seamless integration with caching and minification plugins or to improve above-the-fold rendering performance.

WP Add Custom CSS


Says it has not been tested against recent versions, but it works great still. Simple and elegant solution. Sep 13 2024


Still works great

Add custom css to the whole website and to specific posts and pages.

BlockGlow — Per-block Custom CSS vs WP Add Custom CSS