stylesheet: Improve legibility of app grid, top bar and lockscreen

Sebastian Krzyszkowiakrequested to merge
dos/phosh:bg into main

Darken the background below the app grid a bit and add a subtle shadow on the app grid and top bar when translucent.

This is my attempt at improving phosh's legibility on various backgrounds. I tried to make the changes subtle to not mess with aesthetics of darker backgrounds too much while providing at least some legibility with lighter and busier ones.

Some before/afters with GNOME's default backgrounds:












Merge request reports