If I'm not mistaken can't change the textures in game...
But why this question?
i think this can't exist .
It does, the champion default skins are client based, not sure about the map.
It does exist but very hard to do.
My best advice is to check out the leaguecraf*****m forum and it's modding section which is a pretty good start for modding anything in league.
@Garen is right LeagueCraft is probably your best bet anywhere now.