Multisite Issue - When I enable Multisite, my custom theme does not work completely, some issue with

I am using Hexeris Rocket Theme. When I enabled multisite and changed all the necessary files such as htaccess and wp-config.php then my theme does not work properly, i.e, everything is fine except backgrounds, I just get white background. I tried to find errors but could not find. I know this is unrelated to your plugins and themes, but I am asking you because I want to use your Prosite plugin and I know you guys are experts with multisite. Please help with this.