I have created a project with the ScrollViewer, ButtonChrome and RadComboBox attached. The small button does not go yellow now. Note the ButtonChrome that renders the drop down button had its RenderActive property template binded to IsMouseOver of the RadComboBox. I removed it. Also I put Keys on the ScrollViewer, ButtonChrome and RadComboBox so I can use them in the MainPage.xaml so I plugged them in the CustomComboBoxStyle. Finally I edited the width of the vertical scroll bar items to use MinWidth of 12. You can further edit the Control Templates of the scroll bar. I have also changed some Corner Radiuses. Further you can change the color brushes to make the design look closer to your screenshot.
I am sorry for the delayed answer but I hope the attached project would be of help.
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