Only lexical elements that are identified by the upstream Lexilla lexer can be styled by Geany. AFAIK no Lexilla lexers look at the content of comments other than recognising Doxygen and Javadoc inside document comments, otherwise considering them totally for human not machine consumption. You could ask on the [Lexilla issues](https://github.com/ScintillaOrg/lexilla/issues), and if its added to Lexilla can be mapped in Geany styling.