Add a "devel" CSS theme for our demos

It's useful to know if you're running the demos from an unstable version
of GTK+, so we should use a CSS fragment that updates the visual
identity of gtk-demo and gtk-widget-factory depending on the version of
GTK+ they are running against.
4 jobs for devel-theme in 31 minutes and 25 seconds (queued for 2 seconds)
latest
Status Job ID Name Coverage
  Build
passed #31118
fedora-x86_64

00:06:20

passed #31119
win32
msys2-mingw32

00:08:13

 
  Flatpak
passed #31120
flatpak:demo

00:23:11

passed #31121
flatpak:widget-factory

00:22:46