Youre not allowed to modify parts in any way, in such that any part that you change which results in that part or a connected part breaking, it will not be covered under the warranty.
I cant realistically see changing the tint of the glass as a warranty issue or affecting any other car part in any other way though.
But seeing as many manufacturers offer it as a service, and you could argue that you are preserving the upholstery!! I wouldnt think it was a problem as long as its a legal tint that is!