Thanks for the swift reply!
I am very weak on CSS, but with your suggestion I found this article on the rz-tabview-panel class.
I put the CSS Selector in the site.css file:
.rz-tabview-panel {
padding-left:3px;padding-top:3px;
}
which does exactly what I want, when seen in Radzen Blazor Studio. But when the app runs in the browser, the problem persists. I will learn more about this myself, but if you have a quick fix, of course I would be happy to hear it 
Cheers, Henrik