Changed Geany's locale to LANG=C to make this work properly (it didn't suggest CSS filetype on top of the list whatever command I tried in my locale) but it's OK.
The plugin uses fuzzy matching on the menu item labels, so you need to type something that is there. E.g. in French locale, it's *Feuille de style en cascade*, translated from the "Cascading stylesheet". You can check on your locale and see.
Also, Commander matches the whole menu hierarchy fuzzily, and splits at words, which means that you can match e.g. *Objective-C source file* with `dsfoc`: ***D**ocument **S**et **F**iletype Programming Languages **O**jective-**C** source file* or the like.
Ultimately you'll find the short strings you like and that work best in your locale.