If you wish to apply Theme global, not only for specific controls you should set the ApplicationTheme:
StyleManager.ApplicationTheme = new MetroTheme();
Do not forget to add Telerik.Windows.Themes.Metro.dll assembly to your project.
If you do not want to use Blend you can get the same styles from the RadControls installation folder on your machine. There you will find a Themes folder. Drill down to the xaml file that corresponds to the control and its theme and you will have all needed resources for customizing the control manually.
All visual states of RadComboBox are the visual representation of the ButtonChrome control. In the attached project I sent you all needed resources for customizing the ButtonChrome. To change highlighted state of RadComboBox you need to modify following Brushes