Custom skins

It's very much a grey area. Of course Riot doesn't want you to use any third party programs that change the way their own client works but some custom skins are too good to pass up.

But the most important grey area is the fact that there are custom skins which make you look amazing, there are custom skins which make you look retarded or goofy, but then there are skins which alter animations or colors which give an advantage that otherwise wouldn't be there.

I'm sure Riot wouldn't mind you using goofy or amazing custom skins, but because some players use custom skins to abuse the potential advantages, Riot cannot truly make custom skins an 'official approved open source' thing.

I know that doesn't answer your question but I hope it gives you insight on topics such as custom skins.

I disagree with NSDBL. Almost all programs that mess with internal files like skins/voices/etc are prohibited. It's not really a grey area, you risk getting banned if you install such programs.