Hey.
Just to give you an update this thread. I've spent the last week looking into the code and trying to get a 'dark mode' variant to work, however I've had no luck. I've been able to change the GUI and font colour/style. But I haven't been able to work out how to give the player the choice to change it themselves. I'll try looking into it more, yet I think this may be out of renpy's capability.
Sorry, I did try. :( (The only other option I can do, is have a set dark mode GUI in the game files, in a separate folder; and then have the player extract/rename and replace the original with these.)