In my initial post I tried to cover the theoretics behind Qt styles. (I'll add more links to references and tutorials as I find them.) Now it's time to get our hands dirty.
The easiest and fastest way to achieve results is to use an existing theme as a template and build upon it. JohnFredC and oops66 have paved the way for the rest of us. However, there's still some work to be done.
Here are some samples for your dissecting pleasure:
For the last two years I was hoping for QtStyles.net to become a one stop shop for all things QSS. Unfortunately the site never took off and seems to be abandoned now