Is your feature request related to a problem? Please describe.

I faced the issue of not being able to write CustomCSS settings and solved it in #182.
However, if the extentsions directory has user data, it becomes more complicated to manage. Because the extensions directory is managed outside of the application, but the user data is variable data that the application writes.

Describe the solution you’d like

I think CustomCSS settings should be stored in the database or in each user's data directory, because they are user data.

For example, Readable stores settings in data/users/<username>/config.php.
If it needs to be saved in an independent file, data/users/<username>/custom.css would be better.

Is it a technical difficulty to store CSS in the same way as general user data because it is a file delivered over HTTP?

Describe alternatives you’ve considered

Additional context