I don't think just changing the text color will work. There is an "Effect" property for ScaleLine and MouseCoordinate, so maybe we can set it there?
The scale line in MapSuiteGisEditor sample has an outline, so there should be a way to set it:
If that doesn't work, is there a way to set the background of ScaleLine and MouseCoordinate to white? I tried setting the "background" property, but there were no effect.