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

<ResourceDictionary
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" 
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:Microsoft_Windows_Themes="clr-namespace:Microsoft.Windows.Themes;assembly=PresentationFramework.Aero">

    <LinearGradientBrush x:Key="ButtonNormalBackgroundBrush"
        StartPoint = "0.5,0"
        EndPoint   = "0.5,1">

        <GradientStop Color="#C10099FF" Offset="0"/>
        <GradientStop Color="#C16699CC" Offset="1"/>
        <GradientStop Color="#C1006699" Offset="0.49"/>

    </LinearGradientBrush>

<ResourceDictionary/>

Now i want to get LinearGradientBrush from ResourceDictonary and apply it dynamically to a button as background color in wpf.

 BtnGetBrushes.Background = Brushes.Green;

I want to apply the above color instead of this(Brushes.Green). what should i do for that ?