I have noticed that the themeroller produces buggy ui.base.css files when creating custom themes. As a rule in ui.base.class you have a set of about eight @import lines (resulting in a file size of ca. 1kB), but in the corrupt files the last seven lines are repeated countless times, resulting in 5MB+ files (causing Dreamweaver to hang). I get the same behaviour with Firefox 3.0.9 and Internet Explorer 7.