Support accent color tint

[jumpyvi]

Problem

Gray is nice, but a little splash of colors is always better!

Proposal

I would like for apps (and maybe titlebar/appmenu) background to be tinted with the current accent color.

Prior Art (Optional)

Some exemple for adwaita/qt

On adwaita:
https://github.com/pakovm-git/Adwaita-Accent-Tint video
https://github.com/n3thshan/adw-tint-kde
matugen also does this by default on dms i think : https://github.com/InioX/matugen

[jumpyvi]

It should probably be opt-outable, since it could cause accessibility issues, but not sure it would be the case with a ~7% tint like with adwaita-accent-tint