Really the only tab style that is designed for white is the cnn style.  The rest all use transparent PNG images, so the tabs will look grey-ish with a white background.

You might be getting Flexibility 2 confused with the original Flexibility.  You can tell the difference because the original theme usually has a black background behind the tabs and they don't raise up when hovered over.  In Flexibility 2, it looks best if you change the nav tab background, the hover state, and the nav bar background to be all the same color.