Outdated and thematic skins are stored in the DevExpress.BonusSkins library/NuGet package. These skins must be registered before you can apply them. To register bonus skins, check the corresponding setting on the Project Settings Page, or call the Register method on application startup:
How to: Add and Customize Toolbar Skin Selectors
How to: Add and Customize the Ribbon Skin List and Skin Gallery
How to: Build a Custom End-User Skin Selector
Skin Palettes