I had no idea about the importance of consistent lighting effects on all elements
Traditional MS Windows light-sourcing is from top-left so shadow-casting is to bottom-right.
If you do something such as to reverse this then public perception/interpretation will be that the object is concave rather than what may have been intended to be convex.
If the sourcing is inconsistent [a not uncommon fault] then the success/popularity/appreciation of the skin will suffer greatly.
Remember...we're in a 2D 'world' attempting to infer a 3D one, so not only must the light and shade 'read' correctly also button-states [static, pressed, etc.] need to indicate 'movement' through that same consistent 'expected' light/shade...