I am very new to Geany, and found that it did not support syntax highlighting for Pug, an HTML templating language I use in a number of projects.
Unable to find an existing project for Pug support in Geany, I have implemented a very rough and ready version and released it here:
https://github.com/wgmyers/geany-pug-support
Pug syntax is a bit odd and not really like anything obvious like C, HTML, Markdown etc, so I think that do to it properly would involve writing a lexer in C++, which is currently beyond my abilities. For now, this avoids having no syntax highlighting at all.
I hope this is helpful to someone (and if there was an existing implementation that I failed to find, my apologies, and please let me know where to find it, as it is likely better than mine is...)