On opensuse tubleweed I use Xorg and Awesome WM.
Gnome is installed but I rarely use its session.
On https://bugzilla.suse.com/show_bug.cgi?id=1210065 is reported why I removed
xdg-desktop-portal-gnome from my system.

I like dark theme but the three, most used by me, gnome packages as nautilus,
gnome-calculator and gnome-clocks, perhaps evolution mail too, refused to apply
dark mode even if it was set by tweaks, gconf or other means. The only way, to
have dark mode in these, was to first log in gnome session, log out and finally
log in awesome wm. Some unknown variable, set in gnome session, manage to pass
in awesome wm session and the above three packages run in dark mode.

On logging directly in Awesome wm, dark mode was impossible for these three
apps. Either log in directly on awesomeWM, where dark mode does not apply, or
log in AwesomeWM after log in and out of gnome-session, where dark mode
applies, the variable GTK_THEME is empty. BUT
upon entering a line of

export GTK_THEME="Adwaita:dark";

in ~/.profile or ~/.zprofile for zsh, dark mode is applied always in these
three apps.
However, the setting of GTK_THEME variable cause the crash of the two timers,
that gnome-clocks include, upon clicking on start timer icon.
I removed the above offensive line from .profile file to have working timers in
light mode and I hope that this bug? could be fixed and gnome-clocks will work
in dark mode as gnome-calculator does.