As already noted, if you're going to build your own interface, you'll have to...build your own interface. By drawing it from scratch. There are plenty of tutorials in the Qt documentation that can guide you.

However, note that this generally isn't a good idea. People get used to their native interface, and get disturbed when it stops working the way it usually does. It's better to focus your energies on ensuring that what your program actually does, it does well, and leave the user interface issues to the operating system.