use iconFill:'black' as the default for all button icons, #430
use iconFill:'black' as the default for all button icons, #430

Signed-off-by: Chris Malley <74b2712074b149d140079dbd7e875955d192e617@pixelzoom.com>

phetsims/scenery-phet,phetsims/scenery-phet,phetsims/scenery-phet