Update theme at runtime

[Dimchikkk]

Currently theme change requires app restart, let's write bevy's system that query all UI elements and updates colors/sizes based on user theme change at runtime.

[Dimchikkk]

Another approach: destroy world and re-run setup code on Theme change. Something like this: https://gist.github.com/tigregalis/b1cde096d917c945a840e33a53619752