Create a new Windows Application and drag a RadControl, i.e. RadButton, from your Toolbox to your form.
Create a new custom theme and save it somewhere on your hard drive (save it as XML files).
Right-click the project node in the Solution Explorer, choose Add >> Existing Item and add the custom theme file. This will automatically copy the custom theme file in your project's directory.
Add a RadThemeManager to the form.
From its Action Menu (click the Smart Tag to open it) choose Load Theme from file, then browse to the theme file in the project directory and select it, click Open.
Again from the RadThemeManager's Action Menu, choose Edit Themes, set the ThemeLocation property according to the following pattern: "~..\..\ThemeFileName.Extention", assuming that the theme file is in the project directory.
Now apply the theme to the RadButton.
If the form with the theme manager will be opened multiple time in the life cycle of the application, the theme manager needs to be diposed explicitly. A suitable place to call its Dispose method is the Closing event of the form.