I agree with @codebrainz. It might be harder to do than e.g. the tab labels, because I don't think it's possible to set names/classes on treeveiw cells, but it's likely possible to still query the theme.
As @codebrainz mentioned, it makes it possible to play nice with the theme, and even if we didn't re-use some style that already exists (even though it'd be likely better if possible), because a theme could (theoretically) provide the Geany-specific bits itself to blend perfectly.