Ahh xD This is an old trick of mine I first ever used on the Coca-Cola Truck. Red netherbrick stairs and slabs will be introduced in 1.14 but I didn't want to wait so long until I can use them. So, what I did is I gave bricks the same texture like red netherbrick, which causes brick stairs and slabs to look like red netherbrick stairs and slabs. That's why all the red netherbrick stairs and slabs look like brick ones in the PMC World Viewer. It really only is a texture change ^^
But it solved my problem of not having access to red netherbrick stairs and slabs, although I know for sure they will be part of the game eventually.