The Eseries blog has a great solution for a common problem with S60 themes: the inability to install them because of the smartphone's insistence that their "certificate" has expired.

If the error message reads "Certificate error. Contact the application supplier," all you have to do is set the Application Manager to accept all kinds of Symbian apps, even those that weren't signed by Symbian.

If you're faced with "Expired certificate" instead, it's a simple matter of setting your smartphone's date one year behind, installing the theme, and changing back the date.

Check out the full post for more info and screenshots.