Gtk 4 is the version used in GNOME 40 onward, and aims to replace all other GNOME themes, to widespread unhappiness and discontent. At the end of the 1990s, Xfce was rewritten from XForms to Gtk 2 ...

The Shell theme customizes the look of the GNOME Shell itself, which includes the top bar, system menu, and notifications. The Legacy Applications Themes apply to the majority of GTK-based apps ...