It would probably be easier to apply your customizations from scratch. Alternatively you can keep them in a separate CSS file so you can still receive upgrades and fixes to the themes.
If this is not an option you will have to change the prefixes of the css classes manually -
-
.ui-is now.rz- -
.piis now.rzialso.pi-is.rzi- -
.cardis now.rz-card -
.fais now also.rzi -
.menuis now.rz-menu -
.navigation-itemis now.rz-navigation-itemand.navigation-item-is.rz-navigation-item- -
.panel-menuis now.rz-panel-menu -
.context-menuis now.rz-context-menu -
.linkis now.rz-link