[_bxg1]

It's OS-level. macOS, iOS, Android, and I believe Windows 10 will all send this signal now to whichever (modern) browser you're using.

[lol768]

As will Linux/Gnome.

[skykooler]

Any idea how to set this on Ubuntu? I have what I assume is the dark theme - title bars are white text on dark grey background - but this website displays a light theme in Firefox.

[lol768]

It depends on your Gtk theme, I think. If it's marked as dark, then Firefox will pick that up and apply the CSS accordingly. I use Yaru-dark.

You can also force Firefox, via an about:config setting to act as if you were using dark mode.