The brush that will used to paint the background of this control.

To set a solid color as background, consider using the Color property instead.

This property is automatically set if you put a brush inside the control, e.g.

<Panel>
    <LinearGradient>
        <GradientStop Offset="0" Color="Black" />
        <GradientStop Offset="0" Color="Black" />
    </LinearGradient>
</Panel>