I personally would not tint a car I'm leasing. Since you're just borrowing it, you don't want to throw extra money into it... right? You have to see if they'll allow it, they might not. Did you look over the contract, did it say anything about aftermarket/mods?
This is what I'd do. Go to your dealer, and see if they will throw it in with the lease or if they'd do it for a tiny fee.