Make changing a code theme background easier #814

I was looking at using the presenterm catppuccin theme with bat's catppuccin code theme, the problem is that both have the same background colour set. If I want to change this as a user I currently have to:

• Copy the .tmtheme, edit the background colour, use patched version as normal
• Extend the presenterm catppuccin theme and change the background colour

(not counting using a transparent presentation background)

This also means that if you wanted to edit the default presenterm catppuccin themes to use their corresponding code highlighting theme, it would be a pain.

Hence I propose modifying the code.background to optionally support giving a colour. WDYT?