Extendig style sheet in jupterlab 3

I am working on jupyterlab 3 and want to add e.g.

.boxed {
border: 2px solid black;
margin: 10px auto;
padding: 10px;
border-radius: 10px;
}

and stuff like this to the class sheet. Is there a simple way or do i have do figure out writing a markdown extension?