WIP: Customizable Theme

Closes (the already closed) issue #73 (closed)

This is almost finished now and ready for review. There are still a few bugs waiting to be fixed in the app-theme-engine library, but otherwise it's good to go. If you find any issues or if you have any other suggestions just comment here and I'll try to update the merge request as soon as possible.

PS: I tried to split it in as many commits as seemed possible, but there are still some commits that are rather unorganized, so sorry about that!