Earlier quoted context omitted.
You might be surprised how far contrast (and cursive, bold, etc) can get you. Eg: https://github.com/fxn/vim-monochrome edit: Also, and probably a lot more controversial, code structure matters a lot. No syntax highlighting will save you if your code consists of opaque blocks of text.
My favorite part of that example is the language it's written in :) I'm sold!
As stated above, I think that how the code is structured also heavily influences readability. Maybe even more than syntax highlighting.
Case in point: I was fortunate enough to work on a Haskell project some time ago ... and one of my colleagues from another team at asked me how I manage to make my code look that concise. No secret there, it is just pretty easy if the language is that expressive.