Starting sometime on April 2, 2009, any time a user tries to download a custom theme, the download fails. Reproduced in FF3 & Safari 3 on Mac Leopard, FF 3 and IE 7 on WinXP.
To see error:
1. Go to http://jqueryui.com/themeroller/ and change at least 1 theme parameter (text, color, etc.)
2. Click the orange "Download theme" button, this will direct you to the download builder page
3. On the download page, don't change anything, just click the gray "Download" button
4. Once clicked, the page blanks out completely. The URL is http://jqueryui.com/download at this point.
5. If you hit the Refresh button, a download will start in Firefox and Safari but if you open it, you see an error stating that the zip is corrupted. In IE 7, when you refresh the page it dumps a long page of characters into the browser window and does not start a download.
If you select a preset theme from the TR gallery, the download will work correctly so this only affects custom themes.
A bunch of comments on the UI discussions have mentioned this issue: