WPF在c代码中更改工具提示背景

假设你有这样一个按钮:

    <Button x:Name="btnOne" Content="How to grow big and strong?">
        <Button.ToolTip>
            <ToolTip>
                Eat your veggies
            </ToolTip>
        </Button.ToolTip>
    </Button>

    var tip = ToolTipService.GetToolTip(this.btnOne) as ToolTip;
    tip.Background = new SolidColorBrush(Colors.Magenta);