Internally we use a github based style which people like because it looks good and because github. pygmentize also has bunch of other built in styles which you can list with pygmentize -L styles. Some of them are minor variations while others would clash badly (dark background).
I looked into adding this to PhutilPygmentsSyntaxHighlighter but ran into trouble since the CSS style definitions look like they are tied into the rest of remarkup-land. For comparison cgit just dumps a big old <style> block http://git.zx2c4.com/cgit/tree/filters/syntax-highlighting.py?id=v0.10.2 Was not sure how to structure it here since passing along CSS isn't part of the contract for PhutilSyntaxHighlighterEngine.