How do you add mods to league of legends on my mac?

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wait what? is modding illegal? I see streamers do it all the time

Depends, most are harmless, like adding skins which only you can see, but it's also possible to remove or alter files to give you some advantages, like seeing stealthed champs. I'm pretty sure you only wanted to change your hud's look though, so I doubt there's any problem with that. (Don't know how to do it though sorry)