You can do it one of two ways. The first way is to edit the profile object in GuiControls. This has been documented
here
The other way is to download a Windows theme (.wba) and use that as a style.
The former is the method you would have seen most often as the latter method isn't widely used (or known about). Note that these profiles/styles will only effect GuiControls. You can find a full list of GUI controls
here