css: Tmp message background based on theme_bg_color

Fix #275
3 jobs for master in 24 minutes and 37 seconds (queued for 14 minutes and 51 seconds)
Status Job ID Name Coverage
  Test
passed #53771
flatpak

00:17:20

 
  Deploy
passed #53772
pages

00:07:17

passed pages:deploy

00:00:01