How to customize the theme colours and background

### Discussed in https://github.com/Rezonality/zep/discussions/80

<sup>Originally posted by **totalgee** February 25, 2022</sup>
1. Is there a way to customize the theme without changing the Zep library itself?   I'd like to modify the colours for keywords or identifiers (taking one of the default themes and tweaking a few values).

