Possibility to add custom CSS/JS to own Instance

It would be nice to have the possibility to add custom Styling to a Mobilizon Instance without having to modify the Source.

This could include a hierarchy, where you can add CSS to:

1. The whole Instance
2. A group
3. A single event

On the Instance/Group level it can be used to add the branding of the organization, on the event level it can be used to add effects/styling for special occasions (e.g. a halloween party having a black/orange scheme)

Another part of this Feature would be the possibility to add site-wide JS to easily extend the UI without manipulating the source