lastsplash (a11n)

When WordPress introduced Full Site Editing (FSE) with block themes, the familiar “Additional CSS” option was moved from the Customizer to the Site Editor’s Styles panel. This plugin brings back easy access by adding an “Additional CSS” link directly to the Appearance menu.
Features:
<ul>
<li>Adds “Additional CSS” link under Appearance menu</li>
<li>One-click access to the Site Editor CSS panel</li>
<li>Only appears when using a block theme</li>
<li>Lightweight and simple – no settings needed</li>
<li>Works with any block theme</li>
</ul>
How It Works:
Simply activate the plugin and you’ll see “Additional CSS” appear in your Appearance menu. Clicking it takes you directly to the CSS editing panel in the Site Editor.

Additional CSS Shortcut

Adds a quick-access link to the Additional CSS panel in the Site Editor for block themes.

Alan Fuller

The new generation og WordPress themes are known as Block Themes, sometime referred to as Full Site Editing (FSE) or Gurenberg Themes.
These themes disable the older customizer, on the presumption that all styling can be done via the editor.
Unfortunately this is not so, and some styles still need custom CSS if you want to perfect your designs.
There are several plugins that allow you to add additional CSS however there is no need to add another plugin as the ability is still within Core WordPress, in the form of the customizer.
All it needs is it to be switched back on – no bloat- just a switch and you can then access the customizer and add additional CSS.
This is a really lightweight plugin that adds back the customizer when hidden by block themes.
The customizer is useful for block themes mainly because of Additional CSS.
Whilst it is still available for block themes, it is hidden. This plugin simply brings back the links.

Add Customizer Back For Block Themes

Nice, simple, and does exactly what it says. 

It's getting the job done.

Simply adds back the hidden customizer for Block Themes such as Twenty Twenty Two and Twenty Twenty Three

Additional CSS Shortcut vs Add Customizer Back For Block Themes