CSS fixes for dark themes

This fixes the appearance of some elements in the add-ons section when using a
dark theme, like Adwaita-dark, as described in #265. The fix bases the
background color on '@theme_bg_color' to account for the themed text color
contrast for nomal context, the warnings now have the text color hardcoded to
the value of '@theme_fg_color' on Adwaita (A.K.A. black).
1 job for wip/daveol/fix-darktheme-colors in 5 minutes and 47 seconds (queued for 5 seconds)
latest
Status Job ID Name Coverage
  Build
passed #62835
build-gnome-software

00:05:47