Add options to set color theme globally

[fabianchua6]

Summary

Would be cool if we were able to change the accent or the colour of the theme somewhere!

Motivation

That way it makes it more customisable and maybe unique for each individual.

[kartikcho]

This was something on my mind too but would require a bit of rework. I'll add it to the list though. Thanks!

[kartikcho]

Closing this as it is a duplicate of #322 (although this is the original)
This is still definitely on the list!