When you click the Preview theme button in Microsoft SharePoint Server 2010, the theme may not be previewed. Additionally, the cascading style sheet of the page may display incorrectly.
This problem occurs when you do not have designer permissions on the root Web site for that site collection or when there is a Turkish letter "ı" (the lowercase i without a dot) in the name of the theme.
To resolve this problem, use one of the following methods:
For permissions issues, grant yourself designer level permissions or request designer level permissions on the root Web site.
If the name of the theme contains the Turkish letter "ı" (the lowercase i without a dot), rename the theme without using this Turkish letter.