I found several methods to change the lines and the area between the white squares. But I could not find a method to change the size of the white squares. I would like to decrease the size.
The only thing I could find on Google did not sound very promising:
https://github.com/bhaskarvk/leaflet.extras/issues/80
Sorry, I just found out that I had asked this question before.
https://vaadin.com/forum/thread/17345799/how-to-style-lpolygon-in-polygonfield
Now I tried out the suggested solution and it works (I added both css styles).