Theming

[Maingron]

Add a proper theming engine so people can create and install custom .theme.zip-files.
The standard theme should be reworked, too.

[Maingron]

This should be prioritized. We need a proper theme-"manager".

Maybe create a folder which contains all themes and a setting which lists all the themes and applies the selected theme.

Maybe the applied theme(s) can be stackable like Minecraft Resourcepacks are. This could result in something like this:
4. Oval buttons
3. Orange borders everywhere
2. myCustomTheme
1. Default MainOS theme