Can I use custom skins ?

While use of third party skin programs is not against any rule, a lot of these programs also have aspects of them that do break rules and can result in permabans. Personally I wouldn't even tank the risk.

This has come up a lot recently, and while you can use them, it's use them at your own risk, per this red.

https://boards.na.leagueoflegends.com/en/c/help-support/VA5N7HU5-custom-skins

While use of third party skin programs is not against any rule, a lot of these programs also have aspects of them that do break rules and can result in permabans. Personally I wouldn't even tank the risk.