ThemeManager.ThemeName = Themes.Dark;
Customize Applied Theme
To customize the applied theme, use the ThemeBase class. It provides you with various customizers that allow you to change a theme's appearance and apply it to the grid. You can use these customizers to change background colors, fonts and others attributes of the desired grid element (header, group row and so on). For more information, see the How to: Customize Themes document.