Draft: enable automatic dark-mode switching on macOS
This merge request adds support for automatic dark mode switching on macOS if the default colour theme is selected.
I created this as a draft as there is still a bit of duplicated code which might need to get cleaned up, and to get some initial feedback :-)
Before this patch
Apps like Kate that make use of the colour scheme manager don't get changed into dark mode.
After this patch
Switching between light mode and dark mode works as one would expect.
(I have no idea why most of the icons don't change, it could be because something's amiss in my local setup which is a bit messy but it mostly works well enough for me :D)