Allow changing the background color for Pie, Bar and horizontal bar faces

In some cases it is desireable to have the same background color used
for several faces. Since we do not want to change the current defaults
too much, we instead allow setting a custom background color that can be
reset back to a theme color.

BUG: 437275
2 jobs for !188 with work/ahiemstra/backgrounds in 1 minute and 37 seconds (queued for 20 minutes and 52 seconds)
latest merge request