You've probably heard about the layers of design, a way to model the idea that for example, to design a web app you need different designers, or you need the same designer, with different design' skills.
Each layer builds on the previous one, starting from the proposition layer.
People usually think about the Appearence layer as the one about beauty. Graphics.
But the interaction elements' look should first serve their purpose. It's about graphics in that it's about form, but it's not free form, it's a form constrained by function.
There's a layer between the Interaction and Appearance layers. The "Inter-graphics layer" maybe?