I had pending for weeks to theme scrollbars of controls of my MZ-Tools add-in on modal forms (Visual Studio 2012 automatically themes scrollbars of controls on toolwindows) and finally yesterday I decided to devote some time to the investigation. Here is the answer:

HOWTO: Theme scrollbars of controls on a Windows Forms modal window from a Visual Studio 2012 add-in.
http://www.mztools.com/articles/2012/MZ2012026.aspx