I'm afraid that's not possible. We have a lot of elements and classes that require specific styling and that cannot be done with just bootstrap.
We have, however, a theme based on Bootstrap that you can use: https://docs.telerik.com/blazor-ui/themes. You can see it in action in our demos through the theme chooser at the top right hand corner (see the screenshot below).
You may also find interesting this sample app where bootstrap and variables are used with compiled SASS to affect the Bootstrap theme and the site stylesheet as well: https://github.com/telerik/blazor-dashboard.
Regards,
Marin Bratanov
Progress Telerik UI for Blazor