Archive.org oddly enough. Found a bunch of old winamp themes there too. Found the System Shock 2 themes I used back in the day there and themed my k6-2 machine with them.

that's all mostly a broad rip of themeworld.com, one of the last still running outdated theme sites that aren't installer-walled up (and was a buyout of one australian theme ftp site). I don't think there's any other old theme sites left (that aren't trying to be a malware vector).

Other places of interest:

- gamers.org ftp mirror of gamesdomain.co.uk has game-related desktop themes
- old WinFiles.com disc collections (a few ~95-98 era themes, sometimes dedicated a whole CD to)
- If you're an anime freak, otakuworld.com's still up and running and offers a lot of homemade themes (including that one really very good asuka langley one) - homemade to be specific, not any of the commercial theme packs Japan has had for years beyond.
- there's a winamp skin museum but it's very glitchy and requires javascript to turn the gibberish into proper file names (after caching a page instance. modern websites are great aren't they!!!!!!!!) and there's no chronological browse so pick carefully. You'll never know if you'll find some piece of 99 skin history or some 202x '90's aesthetics' brainfart