Logically, it should be a radial gradient with the light end centered on the sun and the dark end away from it. Ideally, all icons should look like they're lit

Logically, it should be a radial gradient with the light end centered on the sun and the dark end away from it. Ideally, all icons should look like they're lit from above for consistency (light end of gradient at top, dark at bottom). Having some buttons lit from above and some lit from below (or some bulging out and some carved inwards) looks terrible.