Furthermore, the schemes were free to rearrange and reshape window chrome as the artist pleased, meaning it was possible to do things like have a titlebar on the bottom of the window or to make the titlebar into a vertical tab that hung off the left or right edges of the window. Heck, it was even possible to make windows take on odd, non-square shapes. The results weren't always practical, but it was a great enabler of creativity, and made for probably the most true-to-the original OS themes on any platform.
The only thing that even comes close is probably WinXP/Win7 msstyles, but even those have significant limitations compared to Kaleidoscope. Some arrangements of Linux desktops come close from a technical perspective, but suffer from the customizability being divided into 500 small pieces, making it a bit arduous to get everything to match up.
EDIT: One other thing — Kaleidoscope had something unique that I have yet to see replicated elsewhere: there was no installation process for schemes. You could keep your schemes wherever you pleased, and switching them was as simple as navigating to the desired scheme in Finder and double-clicking it. Boom, done. No obscure and/or hidden directories to copy things to, no incantations to make them show up in a control panel. More things could use that classic Mac flavor of simplicity.
Those themes were pretty good, but suffered some significant limitations, especially with dark themes and carbon apps. The best OS X themes really looked great though, since they supported full color and alpha.