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.

Łukasz Nowicki

Post/Page/Product-specific custom code allows you to add custom CSS styles to individual posts, pages, or WooCommerce products. It provides a dedicated area in the editor screen where you can write your CSS code. You can also choose whether the CSS should apply only to the single view or also to archive-type views.
A new meta box will appear below the content editor on the edit screen for posts, pages, and products. You can enter any custom CSS there and decide whether it loads only on the single view or also on archive pages like category listings or product grids.
<h3>Requirements</h3>
This plugin requires at least WordPress 5.0 (remember always to keep your WordPress installation up to date!) and PHP 7.4 on your server.

Post/Page Specific Custom Code

Great plugin!

A very useful plugin

Add custom CSS to posts, pages, or WooCommerce products, with optional archive support. Includes a dedicated editor box.

BlockGlow — Per-block Custom CSS vs Post/Page Specific Custom Code