FWIW the GTK apps that don't work properly with an Arc theme on my
machine, aren't more stable after changing to Adwaita from official
repos. So the issues I experience with a few GTK apps, are seemingly not
related to the used theme. It would require debugging, but IMO Firefox
and Firefox clones as well as Claws aren't worth the effort, Evolution,
Sylpheed, Falkon and Google-Chrome do the job as well. I only consider
debugging Gimp and I hope that after upgrading Ardour, it will remain as
stable as it is (not always perfect stable, but good usable).

--
pacman -Q linux{,-rt{,-securityink,-pussytoes,-cornflower}}|cut -d\  -f2
4.19.arch1-1
4.19.1_rt3-0
4.19_rt1-0
4.18.16_rt9-1
4.18.16_rt8-1