Sorry I misunderstood your meaning when you said you installed a Rockettheme template.
My Chrome Developer Tools is showing an error when accessing the Template Manager:
Request URL:http://54.215.222.110/administrator/index.php?option=com_templates
Request Method:GET
Status Code:500 Internal Server Error
A "500 internal server error" is a very generic error that means a URL you've requested couldn't be accessed. The error is actually on the server side and NOT in the Joomla/Theme code. One of the more common occurences that can raise this error are if you've updated your Joomla/Theme installation and are using a newer PHP version but some code references an older version of PHP. Also installing a new plugin, if not well written, could corrupt the .htaccess file. And there are MANY more than can cause this error.
The best way to get a handle is too work with your hosting provider. Start by reviewing your Apache error logs. You could also do some basic debugging by backing up your current .htaccess file to your HDD. Then on the server copy delete all the code and save. See if this resolves the issue. If so then it's one of the lines of .htaccess code that is causing the issue.