Adwaita: devel style

- nightly/development versions of apps should get a styled
  headerbar.

GNOME/gtk!142
4 jobs for <span class="ref-name">devel-styling</span> in 32 minutes and 44 seconds (queued for 5 seconds)
Status Job ID Name Coverage
  Build
passed #74971
fedora-x86_64

00:06:28

passed #74972
win32
msys2-mingw32

00:14:40

 
  Flatpak
passed #74973
flatpak:demo

00:05:59

passed #74974
flatpak:widget-factory

00:05:35