do you get permanent bans if u use custom skin??

You can get banned for anything that modifies the game files. This is used as a blanket rule that covers everything an anything, so something unexpected won't come up and ruin people's day with no real repercussions allowed. This includes any audio and visual changes.

That said Riot understands the way the internet works, and allows their rules to be bent. You won't currently get banned for using a custom skin or custom announcer pack, just keep in mind that you are technically going against their wishes by doing so, and they are capable of changing their minds at any time.