Where is the system-wide dark mode setting in Linux? I'm using the Raspberry Pi version of Debian, which has both GTK and KDE programs, as well as bygone era programs like xpdf that still work just fine. If I wanted to tell apps that I prefer Dark Mode, where would I add that setting?

Unlike e.g. MacOS, there is not just one setting.

1. Switching to Xfce will make life easier than struggling with whatever Raspberry pi OS is imposing (Lxsession or some such).

2. Go to: Settings -> Appearance -> choose Adwaita Dark theme

3. Go to: Settings -> QT5 settings -> GTK2 theme

4. Firefox: Choose the dark theme.

5. Ungoogled Chromium should also have a dark theme.