Custom skins

I saw on some youtube videos people having custom skins, fonts and clothes. I was wandering if modyfing the game like this is allowed. The idea of having custom skin seems pretty cool. I'm of course not talking about using it to gain an advantage or exclusive skins that were obtainable but now aren't, I was thinking about it in the direction of recolored weapons of the damned. This seems pretty fun but just wanted to know if it is ok to do or not