AppDelegate - didFinishLaunchingWithOptions 에서 폰트 변경 코드를 입력한다.
func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?) -> Bool {
// 탭바 폰트 설정
let appearance = UITabBarItem.appearance()
let attributes = [NSAttributedString.Key.font:UIFont(name: "폰트 이름", size: 폰트사이즈)]
appearance.setTitleTextAttributes(attributes as [NSAttributedString.Key : Any], for: .normal)
}