How to fix Ultimatum update since it breaks sites in the network?

I tried to update the Ultimatum Theme from version 2.3.7 to 2.5, however when doing so, it broke ALL of my sites (on the Multisite) running this theme. I requested support from them but all they told me is to “try regenerating the CSS”. However I don’t know how to do this or if this is in fact the solution to the problem.

For now I’ve restored the sites to a previous version before the upgrade but I would like to upgrade to use the new features.

> Please help provide a solution for this problem. I was even thinking if there was a way to prevent the current sites from updating the theme but for the rest of the network to update.

Let me know. Thanks in advance.