Background change using LinearGradient

Background change using LinearGradient

I'm programming in WPF(C#). I'm trying to define some thing to change skin
of program.
I use this code:
<LinearGradientBrush x:Key="AquaGradient" EndPoint="0,1"
StartPoint="0,0">
<GradientStop Color="#FFF6F6F6" Offset="1"/>
<GradientStop Color="#FFEAE8E8" Offset="1"/>
<GradientStop Color="#FFDCD9D9" Offset="1"/>
<GradientStop Color="#FF46AFBF" Offset="0.5"/>
<GradientStop Color="#FFE0DDDD"/>
<GradientStop Color="#FF4EDAF0" Offset="0.018"/>
<GradientStop Color="#FF75C2BF" Offset="0.964"/>
</LinearGradientBrush>
<LinearGradientBrush x:Key="GreenGradient" EndPoint="0,1"
StartPoint="0,0">
<GradientStop Color="#FFF3F3F3" Offset="0"/>
<GradientStop Color="#FFEBEBEB" Offset="0.082"/>
<GradientStop Color="#FFDDDDDD" Offset="0.071"/>
<GradientStop Color="#FF0FFB2F" Offset="1"/>
</LinearGradientBrush>
<LinearGradientBrush x:Key="YellowGradient" EndPoint="0,1"
StartPoint="0,0">
<GradientStop Color="#FFF3F3F3" Offset="0"/>
<GradientStop Color="#FFEBEBEB" Offset="0.082"/>
<GradientStop Color="#FFDDDDDD" Offset="0.071"/>
<GradientStop Color="#FFD0FB0F" Offset="1"/>
</LinearGradientBrush>
<Window.Resources>
how can I assign LinearGradientBrush to a control Background ? some thing
like this:
imageInsertUser.Background = GreenBackgroundGradient;