WPF Resource Dictionary Creation

Question

Could some point out what is wrong with the following style? I'm learning how to style and have found this piece of code that I would like to explore, but it gives me an error saying: Value="{StaticResource HeaderBrush}" - The Resource "HeaderBrush" could not be resolved.

Below is the code I'm using and as far as I can see, the "HeaderBrush" is defined...

Please give me some directions as to what I' doing wrong.

<Style x:Key="HeaderStyle" TargetType="DataGridColumnHeader">
    <Setter Property="Background" Value="{StaticResource HeaderBrush}" />
    <Setter Property="Foreground" Value="White" />
    <Setter Property="BorderBrush" Value="{StaticResource HeaderBorderBrush}" />
    <Setter Property="BorderThickness" Value="1" />
    <Setter Property="SnapsToDevicePixels" Value="True" />
    <Setter Property="HorizontalContentAlignment" Value="Center" />
    <Setter Property="MinWidth" Value="0" />
    <Setter Property="MinHeight" Value="30" />
    <Setter Property="Cursor" Value="Hand" />
</Style>

<LinearGradientBrush x:Key="HeaderBrush" StartPoint="0.5,0" EndPoint="0.5,1">
    <GradientStop Color="#FF6B8E95" Offset="0"/>
    <GradientStop Color="#FF14A7C1" Offset="1"/>
    <GradientStop Color="#FF1E424E" Offset="0.509"/>
    <GradientStop Color="#FF1D4855" Offset="0.542"/>
    <GradientStop Color="#FF1D4855" Offset="0.542"/>
    <GradientStop Color="#FF193A44" Offset="0.526"/>
</LinearGradientBrush>

<LinearGradientBrush x:Key="HeaderBorderBrush" StartPoint="0.5,0" EndPoint="0.5,1">
    <GradientStop Color="#FF1D1D1D" Offset="0.614"/>
    <GradientStop Color="#FF007F96" Offset="0.853"/>
    <GradientStop Color="#FF0AEAFA" Offset="1"/>
</LinearGradientBrush>