i3: change bar bg to match Blue-Submarine gtk
use Blue-Submarrine for gtk3 as well as gtk2
compton: fix double shadow for gtk3 apps

Some gtk3 applications drew their own shadows because of client-side
decorations, and then got another one from compton. Now,
compton detects this and excludes these windows from
shadows.
compton: make shadows gentler (radius, opacity)