I have released a new version of WP-GeSHi-Highlight. It includes a brand new default CSS file which works great with established themes such as twentytwelve and twentythirteen. It contains basically the same CSS code I am using here on my website. One example (Python):
>>> "wp" + "-" + "geshi" + "-" + "highlight" 'wp-geshi-highlight'
Furthermore, I have incorporated all upstream changes from GeSHi, straight away from their git repo. This brings along tons of language updates, even for bash.
For more information about WP-GeSHi-Highlight, you might want to visit http://gehrcke.de/wp-geshi-highlight.