We use Telerik in the company I work for and I've faced the same issue, normally modifying the css of a component from a component library sometimes can be difficult since it can change the behavior of the component (talking from my experience). But the workaround I found to modify any telerik component css is to use within the .razor page (or .cshtml) you are using it.

Let me show an example, I've created a .razor file called Example.razor which has this code

To demonstrate what I said before we are going to use blazor css isolation

As we can see I've created a css class, if we use that css class in the button component the background color wouldn't not apply. But if we use the tag withtin the same .razor file this is going to work

This way yoy can achieve what you are trying to do, I hope this enlighten you. (same thing can sometimes apply to the script tag)