The only thing that would void a BMW warranty is a modification that causes errors or problems with parts of the car covered by the original BMW warranty. For example, switching out the exhaust or switching a turbocharger to a supercharger may void the warranty if it causes a covered component to fail.
However, making the switch to bigger wheels or tires, adding a window tint, or blacking out your taillights wouldn’t void the warranty. Warranties are protected by the federal government under the Magnuson-Moss Warranty Act of 1975, so as long as you don’t damage other components, you should be good to go.