acually in my application, we use the OfficeBlue theme. In our application we have many RadExpanders and with this theme, all RadExpanders are all white. I want to know how I can change the style of the RadExpander to reflect the style of the RadButton (gradient, etc...)
If you'd like to set only the Background and BorderBrush of the RadExpander to match the RadButton style, then you can only edit those settings as demonstrated in the sample solution.
If you'd like to further extend the ControlTemplate of the RadExpander control to reflect all visual states of the button (like being highlighted on a click), you'll need to get the Office_Blue theme of the RadExpander and modify it accordingly. You can find the theme under your installation directory either in the Themes folder or in the Themes.Implicit folder in the OfficeBlue\Themes\Telerik.Windows.Controls.xamlResourceDictionary.