As per the instructions by the responder in this article: https://www.telerik.com/forums/how-to-add-a-class-in-the-tooltip-

I would like to have the ability to add a custom class to a tooltip's container when creating a tooltip. This class should be applied before any animation effects take place. Perhaps it could be a configuration like "containerClass": "myCustomClass". This is necessary because we use different types of tooltips on the site and cannot have one global look for tooltips.