Support custom/user-generated themes

[tconbeer]

Posting does this. See also this.

Honestly it's quite a bit of work, especially to extend the support to syntax highlighting. I wonder how many people would take the time to create their own theme...

[whitcodes]

I've opened a PR implementing custom themes here: #966 I took inspiration from Posting's approach as you suggested, but simplified a lot but not focusing on the syntax-specific highlighting, which i havent had any issue with in my own testing. Would love your feedback.