Website theme requires at least version 5.3 of PHP on server. Make sure your hosting server has at least PHP v5.3. If not, check if you can change it in your hosting dashboard panel. In most cases there is such option, but if not, ask your hosting provider to switch/update PHP on your server to at least 5.3 version. If your hosting provider uses older version and refuses to update it, I would change hosting provider at that point, because it means you are stuck with 8-years-old, not updated and not supported software.
Rescue solution: to get access to WordPress dashboard back (with no PHP 5.3), you have to access server files by FTP, and remove (or change name) of the theme folder:
If you can’t switch PHP on your server, you can use older version of the theme, which is included to the package in the
folder – this version doesn’t require PHP 5.3. But I would use this only as a temporary solution.