Of course you don't need Steam to use custom skins, but we were talking about the Retro skin, which is exclusive for Steam users. So, we do need someone who has the Steam version to upload it for us, so we can use it with out normal retail game.
Yes, you can use it without having Steam. Someone who has the Steam version of the game, could upload his skin.
I would really appreciate if someone would do that. You don't have to post the link here, just PM us.