Many materials have a blank
Select your slab (or mesh, etc) => Object Settings => Floor Plan And Section => Cover Fills
You can either select one from the
list, or set it to use the Surface's fill.
The Surface's fill can be modified under the Options => Element Attributes => Surfaces => [Relevant Material] => Vectorial Hatching
Fills themselves can be customised under Options => Element Attributes => Fill Types
ps. You'll also want to check the
box and select the second option in the
dialogue if you're wanting to blanket replace the cover fill.