Kaleidoscope
Shareware for classic Mac OS that reskinned the entire interface, giving the Mac the themes Apple itself never shipped.
Greg Landweber and Arlo Rose built Kaleidoscope in the late 1990s as a control panel that slipped in between applications and the system's drawing routines. A theme, called a scheme, described window frames, scroll bars, buttons and menus, and users produced thousands of them: imitations of BeOS, NeXTSTEP and Windows, as well as pure experiments in brushed metal and velour. The scheme archives were among the busiest Mac sites of their day.
It filled the hole left by Appearance Manager's withdrawn themes and peaked around Mac OS 8 and 9. Mac OS X had an entirely different drawing engine, and Kaleidoscope didn't make the move. Rose went on to Konfabulator, one of the first desktop widget engines.