When you set dark mode in the current gnome (with support for dark mode), the following will be set:

    org.gnome.desktop.interface color-scheme 'prefer-dark'

Valid values are “default”, “prefer-dark”, “prefer-light”.

    org.gnome.desktop.interface gtk-theme '(your-theme)-dark'

This is the one you have set.

    $ gsettings get org.gnome.desktop.interface color-scheme
    No such key “color-scheme”

Debian 11. Is it because it's old-ish?

Oh, Gnome 3.38. I'm not sure that version even supported dark mode, only dark themes. I won't comment on aging of Debian ;).

Btw, what version of Firefox are you using? Debian comes with ESR, so chances are, that firefox' "ui.systemUsesDarkTheme = 1" still works (it doesn't in newer releases).

I have 102.6.0esr (came with the OS) and I also installed 126.0. Neither of them has has `ui.systemUsesDarkTheme`.

So anyway do you think that if I upgrade to the latest Debian, your gtk command line will work?

It should; it does on the gnome install: https://youtu.be/Vpkz89gN9zk

Oh man that's such a nice feature.

I guess now I have a reason to upgrade.

Thank you!