¶ Problem
The GTK3 Theme I set keeps random changing reseting.
¶ Cause
This due to the Legacy (GTK3) Theme Scheme Auto Switcher extension, it allows the GTK3 theme to sync with the Libadwaita Dark/Light mode, this however isn't compatible with out-of-spec themes such as Whitesur.
¶ Solution
Simply disable Legacy (GTK3) Theme Scheme Auto Switcher from the extension manager
