Switching between 2 themes feels jumpy and disrupting, especially when using a dynamic theming webextension. For example, when switching tabs with the VivaldiFox webextension, there's no color transition, which feels jumpy. A fix for this would be adding a transition for main UI colors such as the frame background and the toolbar background, so the transition feels smooth.
Yes! This would be pretty cool! (But calling this a P5 because it's not actually hurting users.)