You can change the theme of an entire application at design-time or run-time by setting the ThemeManager.ApplicationTheme property.

Application Level Theme

You can change or override a theme on a control level by using the ThemeManager.Theme attached property in the XAML markup. To apply the theme to multiple controls in a single view, simply use the ThemeManager.Theme attached property at the root element in the logical tree (the top most parent of the controls).
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Control Level Theme

ThemeManager supports the most common Microsoft controls and will apply a specified theme to the Microsoft controls. There is a property to turn off this support, just in case you don't want to theme the standard Microsoft controls in your applications.
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Microsoft Controls Support

You can easily extend Infragistics existing themes by creating a class that derives from a built-in theme class, overriding the ConfigureControlMappings method, and adding custom resources that will be applied when the theme is used by ThemeManager.
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Extend Existing Themes

Easily design custom themes by creating a class that derives from ThemeBase, implement the abstract ConfigureControlMappings method, and then provide the necessary resource dictionaries that should be used when applying the theme with ThemeManager.
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Custom Themes