When selecting GDM as the Display Manager, the theme switches to Adwaita and I can’t switch it to anything else using LXAppearance. I can see that LXA does change ~/.gtkrc-2.0 as expected, but somehow the system doesn’t pick it up.
Filing this under i3wm because that’s the WM I use, but this seems to be a bug within GDM or something similar, since using another DM such as lightdm solves this.
Just found a workaround involving these terminal commands:
gsettings set org.gnome.desktop.interface gtk-theme "Arc-Dark"
gsettings set org.gnome.desktop.wm.preferences theme "Arc-dark"
gsettings set org.gnome.desktop.interface icon-theme "Arc-X-D"