As an alternative to editing built-in themes, you can clone them and edit the copies. This allows you to apply your changes to only specific applications, while preserving the visual style of applications that use built-in themes.
As opposed to the predefined themes, Theme Editor does not sign the newly created ones. If your application requires using strong-named assemblies, you should then sign the theme assembly manually.
To create a new theme from a built-in one, follow the steps below:
Run Theme Editor. Open the File menu and click New.
The New Theme dialog will be displayed.
Select a DevExpress theme that should be cloned. Enter the path to the theme project file in the Base theme field, or click the Browse button to invoke the Open File dialog.
DevExpress themes are located in the C:\Program Files (x86)\DevExpress 18.1\Components\Sources\DevExpress.Xpf.Themes\ directory.
Specify a location where the new theme will be created. Enter the path to the desired directory in the Target location field, or click the Browse button to invoke the Browse for Folder dialog.
The new theme will be created in the %TargetLocation%\DevExpress.Xpf.Themes.%ThemeName%\ directory.
To learn how to apply a newly created theme to a WPF application, see Applying Themes.