As discussed on the mailing list ( http://lists.geany.org/pipermail/users/2012-October/008346.html ):
Scintilla version in Git Geany supports SCSS, just set the lexer.css.scss.language CSS lexer property to 1. I don't know SASS and HAML, but I don't think Scintilla have those yet, as Lex said.
reading the related Scintilla posting ( https://groups.google.com/forum/m/?fromgroups#!topic/scintilla-interest/QiC_5vvfXk0 ) seems to indicate that a HAML Lexer was never finished.