[Feature] Custom theme/CSS support

[X1pheR]

Is your feature request related to a problem? Please describe.
I like the service but would like to apply some styling.

Describe the solution you'd like
I'm using the docker image and would like to be able to create a custom theme or apply custom css. Also to change styling and colors of the folder and file icons.

Describe alternatives you've considered
I read that it's possible to apply custom css by changing the index file but since I'm using a docker image I can't.

Additional context
None

[coderaiser]

Are you up for a PR? Right now there is light and dark themes in repository, you can add one more, so it will be bult-in, and you can set it in Config.