Currently, in ThemeBuilder, you can't style DropDownList, which uses groups.

https://demos.telrik.com/kendo-ui/dropdownlist/grouping 

https://www.telerik.com/kendo-react-ui/components/dropdowns/combobox/grouping/ 

Consider the option to expose a template that will allow adding custom styles for DropDownList with grouping.

Requested through t.1645162