 <p>Most names are case-sensitive. It should work if you use <code>lexer_filetype=CSS</code>. Also, I'm not positive, but you might want to name the file as <code>filetypes.SASS.conf</code> even it's not required (not sure), it's somewhat of a convention for custom filetypes.</p> <p>Sadly, if you want to find the available lexer filetypes, you'll have to <a href="https://github.com/geany/geany/blob/1.23.1/src/filetypes.c#L349" rel="nofollow">read the source</a>.</p>