Theme Support for Markdown Reader

[itspngu]

I forgot about the whole markdown renderer colors. [...] I just noticed it doesn't look very good on light themes.

https://github.com/dlvhdr/gh-dash/blob/d25d8101d5717a4e10ae83c2d33ae8d1fb7a3a08/ui/markdown/theme.go

Originally posted by @dlvhdr in https://github.com/dlvhdr/gh-dash/pull/146#pullrequestreview-1040997069