<p>Has anybody experience in building a custom style in Qt? What I have in my mind is a complete new style that affects all kind of widgets. I have seen some examples in the web for a custom combo box. But I have no idea how much time and code it takes to build a "complete" new custom style ... maybe someone can give me a hint.</p>

<p>We think of using Qt 4.3 (or even newer) ...</p>

We've done it in the past (in Qt 3), and it's extremely time-consuming. We had a lot of problems with flickering, redraws not working the way we expected, sluggish behavior, bugs in the Qt implementation. It a lot less straight-forward than it seems, and there's little support or user experience too. Unless you need something really particular (as we did), I'd say it's not worth the trouble.
Other frameworks are supposed to make it easier (some Java-based?), but I don't have first hand experience.