How can you retrieve a Brush from a ResourceDictionary defined in XAML and apply it to an element in code?

Assuming your ResourceDictionary available in the context:

<Button Background="{DynamicResource ResourceKey=ButtonNormalBackgroundBrush}" />

or in Code

button.Background = (Brush)FindResource("ButtonNormalBackgroundBrush");

BtnGetBrushes.Background = this.Resources["ButtonNormalBackgroundBrush"] as LinearGradientBrush;