Addition of custom themes for Manim

[MrDiver]

Some form of implementation of this

• Attempt to implement a theming concept #2015

@behackl and I discussed this a while ago. It would be nice to have some theme based color selection which can be replaced. The idea is to have constants such as FOREGROUND_COLOR,BACKGROUND_COLOR and so on. The specific naming of these constants can be discussed here.

I would suggest doing this with an enum of some sort and then switching the Enum values with a theme selector or something like that!

Example from PR

A light mode theme	A sepia theme	A seagreen theme