ui: add background color picker for windows

[tincho70]

It allows you to visually differentiate windows, especially in configurations for large monitors.

[purrgrammer]

hey Tincho, thanks for the contribution. one issue with this is that the text colors might have poor contrast with the color choices hardcoded here, and makes my job designing the theme way harder since there's lots of combinations. i have started working on a theme system for grimoire, where we can define alternative themes, perhaps we could use that to be able to give alternative colors to windows?

[tincho70]

You're welcome! The pleasure of (trying to) help is all mine 🙂
I had tried these colors and they seemed to work well, but now I think it might be because I have the dark theme on my PC...

I don't know how to work with Blueprint (I understand it's the theme scheme you're using now), and I didn't have time to figure it out. I wanted to offer something for you to iterate on, so I think it's excellent that you're integrating it with your theme system, as I still believe it helps improve visuals on large monitors.