I'm not sure this is worth working to any great degree. You can document the CSS on the main documentation (MCR will obviate the need for the CSS page and separate categories I think) and CSS documentation can be made in the page itself.

I didn’t talk specifically about the documentation, it’s just one of the solutions that will be the most easy to implement.

MCR will probably allow TemplateStyles slots on template pages and documentation slots on any page (including a styles.css page). I'm not sure it's worth the effort to make something temporary until then. (Although, given that 1.31 is an LTS version, backporting doc subpage support to it would be nice...)

Hi! I was about making the same request for wp:fr because all TemplateStyles pages are now listed in Special:UncategorizedTemplates (almost empty on wp:fr).
Adding a documentation would be a good solution as we have a generic template for sub-templates pages self-generating a category.
If it is too complicated, adding a category in the TemplateStyles pages is also a possibility.
Many thanks.

In T200614#4511137, @FDo64 wrote:

If it is too complicated, adding a category in the TemplateStyles pages is also a possibility.

You already can. Place the category inside a CSS comment.

It works, many thanks @Izno!

In T200614#4461256, @Tgr wrote:

MCR will probably allow TemplateStyles slots on template pages and documentation slots on any page (including a styles.css page). I'm not sure it's worth the effort to make something temporary until then. (Although, given that 1.31 is an LTS version, backporting doc subpage support to it would be nice...)

5 years later, no MCR for TemplateStyles, but the issue with template categories still exists. Maybe this is the case for not making a bet on non-existing technologies.