And it wasn’t just theming. Classic Mac OS extensions could do anything from add support for new hardware to overhaul the text rendering system entirely to giving dragged desktop icons gravity and inertia to adding a taskbar or a dock. The sky was the limit, and having a single common target to do any of those things (vs. being split between the kernel and a thousand layers/daemons/DEs/etc) meant that if it could be done, it probably had been.