stylesheet: Improve legibility of app grid and top bar
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:























