Transition from light to dark theme is not smooth and consistent

[bertysentry]

Problem

When switching from light to dark theme (and reverse), the transition of colors is not smooth and consistent. Some graphical elements (borders, text color, background colors) flip to a new value immediately, with no transition.

Specification

Make sure all color attributes affected by a theme change will transition smoothly. If possible, use SCSS features to ensure this without repeating the same transition properties everywhere.

Also, be careful with some OTHER transitions that have a specific purpose, like reacting to mouse hovering on buttons, menu links, etc. These must not be overridden by the transition used for theme changes.