"New" demo? Anyways, yes you can use skins

But unlike S2 licensed racers, you cannot upload them to LFSW, so others won't be able to see your skins. You can give them to your friends manually, though, so they will be able to view 'em.

So, if I buy a S2 license, THEN I can use skins online?

Quote from Blaeza :

So, if I buy a S2 license, THEN I can use skins online?

you can use skins online in demo, but no-one else will see them, in S2 it automagically downloads everone elses for you so you can see theirs and vice versa

With an S2 license you can upload them to LFS World and other players will automatically download your skin, when you join. You can use your skins online now, too, but like already said, you have to distribute them manually.