Workaround: load the themes with ThemeResolutionService: http://www.telerik.com/help/winforms/themes-using-custom-themes.html
Note that while this workaround does appear to work, it means that the designer will not show the theme that is applied. Using the RadThemeManager and the ThemeResolutionService together also causes intermittent freezing. (Seems like some kind of infinite loop in Telerik.) So be sure to remove the RadThemeManager if using this work around.