Try the time ones at 3:14am. This is (I presume) when it changes to something weird. 3:14, of course, is the first digits of pi, so this is why it is chosen (the sweet dreams theme has the greek letter pi in the sky made out of stars at 3:14).
If you can't be bothered waiting to 3:14 am, you can do this to apply the css:
1. Get firefox with web developer toolbar
2. press Ctrl-Shift-E to open the css editor.
3. Open the correct css file. For example, the teahouse theme one is here: http://www.google.com/ig/skins/teahouse/teahouse_3.14am.css
3. Copy the CSS from the file and switch back to the Google homepage, then paste it in the Edit CSS panel under Embedded styles. It shouldn't be too hard to make a greasemonkey extension to do this stuff. (But I don't know how to)