Well, your question is addressing wo different problems:
* Creation of your own widgets (even for the purposes of different look).
Creating widgets is very simple in U++ (if you know how - this was one of basic design principles. I guess for starters, look at "reference/UscCtrl" which demostrates creation of simple widget, implemented in Counter package.
The example even demonstrates how to add .usc script description so that you have the right visual response in LayoutDesigner; note that this is not necessary (you can add your custom widget anyway, you will just see it as empty rectangle in layout designer
* The look of existing U++ widgets.
This issue is in addressed in post-605 U++ by introduction of "Chameleon system" - you can adjust appearance on per-class and often even on per-instance basis. This is still under development, however the system is flexible enough to use XP theming API, which brings native look on Windows for U++....
I plan to introduce some reference examples about chameleon - if you are interested, let me know, I will do it sooner