Geshi seems under maintained, its overall model not very suitable for integration in modern web applications, and with questionable performance.
Looking at various other popular and industry standard syntax highlighters, most feature a generic parser. They make use of grammars to map characters to language-neutral concepts. The output html uses generic class names for themes to use in their stylesheets. Some languages only implement some concepts. Other concepts only apply to a small subset of languages. Once abstracted into what needs (different) colouring, only very few separate entities remain.
TextMate themes provide format for language grammars and css classes. Lots of software has adopted this standard (editors like TextMate, Panic Coda, Sublime Text, and Atom). As well as ports into other languages for use server-side in web applications (e.g. GitHub uses it as well).
- .. (google tmTheme)
- https://github.com/textmate/ (190+ grammars)
- https://github.com/github/linguist/tree/master/vendor/grammars (175+ grammars)
- .. (google tmbundle)