Most allow you to tweak certain aspects, not remove the entire rendering output and start over. I've never found any OS widget kit that allows completely and total control of the widget at the pixel level.
On win mobile if std win forms were not enough, we were able use win api(good old c and pointers). It's highly customizable, but with high amount of hand work.
In gui kits most slogans - "write less, get more". It's boosts development, but with less control.
