Add a transition between light and dark themes

[jzgom067]

What needs refactoring?
The theme transition (or lack thereof).

Why?
Currently, the website doesn't have any transition between light and dark mode. It's instant, resulting in a flashbang when switching from dark to light mode. Please add a transition for this.

Also, some elements have their own transitions defined which will probably be inconsistent with everything else on the site after adding the global transition. These individual transitions should be removed.

There are also some things that should not have a transition, like hover states or availability painting. This will probably have to be discussed on a case-by-case basis as a team.