Comment 0 for bug 1971097

In the Ubuntu session, gnome-control-center has an "Appearance" section which allows to switch from Light and Dark themes (which will apply to GTK4 apps as well as set the preference for GTK3 apps), but that section doesn't exist in the control center in the GNOME session.

This means that while you can still choose alternate themes using GNOME Tweaks, they won't properly apply the chosen variant, and by default setting a dark theme will apply that theme's light variant to GTK3 apps while also not switching GTK4 apps to the dark variant of libadwaita.

You can set the which variant with the command line ("gsettings set org.gnome.desktop.interface color-scheme prefer-dark"), but you cannot do this within the UI from the GNOME session like you can from the Ubuntu session.

My understanding is that this stems from the gnome-control-center being in its 41.4 version, with the light/dark switch backported in for the Ubuntu session but not the GNOME session. That in my opinion doesn't make for a great user experience in the GNOME session.