After discussion with Urs, we came up with a scheme for adding media support.
If we add this to config.php in the themes:
$THEME->mediasheets['handheld'] = array(...);
$THEME->mediasheets['screen'] = array(...);
and if mediasheets is defined, then add an additional stylesheet link to a composite stylesheet which will be CSS just for that media type.
In addition, we should add the parameter
to the existing stylesheets that we already have. This is assumed anyway but it's nice to make it explicit.