Document & simplify deployment as serverwide Gitea theme #1

The theme looks great so far, thank you!

I went ahead and

changed the naming scheme, so inclusion as builtin Gitea theme is simplified
documented how to include the theme with Gitea
fixed a syntax error in the LESS file (but didn't recompile as I don't have the tooling installed)

I would actually prefer to keep it as the Stylus preprocessor, as that is what I'm most familiar with. I saw that it also worked with Less and assuming other preprocessors that support CSS nesting as well (I didn't use much of other Stylus features), so I decided to give people that option.
I probably should have also mentioned that you would need to change the file extension to whatever preprocessor you use.

Ah ok, yeah we can name the source file differently than my suggestion. Any suggestion?

I might just keep it as Stylus, which the file extension for that is .styl

The rest I think looks good, I'll merge

 When making contributions, don't use the compiled version.
 . Download the [`theme-gitea-modern.css` file][theme-file] and add it to your custom folder in `$GITEA_CUSTOM/public/css/` ([learn how to find that directory][doc-dir]).
   - The path has changed in Gitea 1.15, there the file needs to be placed in `$GITEA_CUSTOM/public/assets/css`