How can I get iOS 13 on my iPhone 6

What about it? It was introduced with iOS 13 and it requires that. If you want the Dark Mode, you'll need to buy a newer iPhone model that supports iOS 13. The iPhone 6 is pretty old now and the phone doesn't have enough RAM for iOS 13.

The iPhone 6 can not be upgraded beyond iOS 12.4.4.

Dark Mode is only supported in iOS 13. Without iOS 13, you do not get Dark Mode.

Dark mode requires iOS 13, which is not available for the iPhone 6.