I just noticed that Github did something to code highlighting theme they are using, and for whatever reason, it looks very compelling to me. Is there already an Emacs version of this theme? Is there an easy way of converting the CSS file somehow?
You may give emacsthemes a try, especially the github one.
It is available via melpa (don't miss their getting started if you are new to package management with emacs) ; the version seems pretty maintained - last version here : 20170413.
Don't forget to
M-x customize-themes once installed to activate it.