You're getting that error because that theme is not authoring tool compatible.

To get it upgraded you can either log a ticket in the github repository to see if the theme's author will make the necessary changes - or you can try making the changes yourself.

As with anything open source it's good idea to check how recently it was updated before making a decision as to whether you use that code or not. For anything in github this is easy to do by browsing the github repository.