I cannot get syntax highlighting to work on my Jekyll-powered blog.
The development files can be found here: https://github.com/StevenXL/stevenxl.github.io.
I had the same issue using the monokai highlighting css from the jekyll-uno theme.
monokai
To solve the problem I had to update the CSS, using rouge itself is quite straightforward (taken from the docs):
rougify style monokai > css/monokai.css
There are other themes available, too.