Installing a theme

I just tried to install a new theme in a WordPress site that was already set up with a Socrates theme.

I downloaded the Blogs MU theme and tried to install the theme, but ended up with this error:

Fatal error:
Call to undefined function wp_get_theme() in /home/boyretto/public_html/gardenequipmentreview.com/wp-content/themes/blogs-mu/_inc/functions/options-functions.php on line 10

Can you help please, thanking you.

Roy Bretton