This issue is related to the "themes.json" asset in your guvnor repository. After upgrading your Designer version, that file got updated, but Designer does not overwrite it in your repo (because you could have some custom chagnes in it).
To fix your problem, go to the Globals package in Guvnor and open the themes asset, then archive it, go to the Archive and permanenty delete it. Then open a new or reopen existing business process. This will trigger Designer to install the newest version of this file and your issue should be fixed.
HTH