css: Use group at-rules definition

2 jobs for master in 8 minutes and 13 seconds
Status Job ID Name Coverage
  Test
failed #868291
flatpak
flatpak

00:08:13

 
  Deploy
skipped #868292
nightly
 
Name Stage Failure
failed
flatpak Test
A connection to the bus can't be made
Uploading artifacts for failed job
Uploading artifacts...
gitg-git.flatpak: found 1 matching files and directories

repo.tar: found 1 matching files and directories

.flatpak-builder/build/gitg/_flatpak_build/meson-logs/meson-log.txt: found 1 matching files and directories

.flatpak-builder/build/gitg/_flatpak_build/meson-logs/testlog.txt: found 1 matching files and directories

Uploading artifacts as "archive" to coordinator... ok
id=868291 responseStatus=201 Created token=uuod3kNb
ERROR: Job failed: exit code 1