Document `imbaconfig.json` and its theming capabilities

https://discord.com/channels/682180555286380545/682180555286380726/877896113947033610

> You can add a shared file where you define some css variables @root with your colors, and then refer to those variables, but it is also possible to declare your own color palettes in imbaconfig.json - it is just not documented :stuck_out_tongue:

Start with imbaconfig.json and the CSS-variable/@root theming details described in the issue. Document how shared CSS variables work and how custom color palettes are declared in imbaconfig.json, including enough detail for users to apply both approaches.

Written by the indexing model from the issue text.