Separate light and dark themes into different lists #439

Currently, scrolling through the theme list in a low light room is an exercise in getting flashbanged over and over.

I kinda had a realisation that people will likely be looking for either a light theme or a dark theme, so it would make sense to allow users to select that first?

i.e.

• Which colour mode would you like to use?
  • Light
  • Dark

Then you get shown a list of themes only for that mode.