When selecting a theme within the WPMU interface, I noticed that a lot of my themes didn't display a preview of the site before confirming activation. It popped up the thickbox js window but that window was then blank. However, the theme was still successfully activated.
Upon investigation, it appears that all theme folders must be at the ROOT LEVEL of the themes folder (within wp-content > themes) for the theme preview to display correctly.
Unfortunately, if like me, you need themes to be logically filed in separate folders within this default theme directory (e.g. wp-content > themes > mainsite > Mandigo folder, Kubrickfolder, k2folder etc) then this subsequently kills the preview window.
Can anyone else confirm that they experience this behaviour too. Its most frustrating as all the themes in these subfolders are still selectable and usable within Wordpress. Its just that previews that don't seem to work.
And no, I don't have any spaces or other rogue characters in the directory names that could be causing any issues.
WPMU 2.6.3, MySQL 4.1.22-standard