How to change the checkbox NormalRectangle?

[Coloryr]

<StackPanel.Styles>
    <Style Selector="CheckBox">
        <Style Selector="^ /template/ Border#NormalRectangle">
            <Setter Property="BorderBrush" Value="White" />
            <Setter Property="CornerRadius" Value="20" />
        </Style>
    </Style>
</StackPanel.Styles>
<CheckBox
    Content="{Binding Title}"
    Foreground="#FFFFFF"
    IsChecked="{Binding IsCheck}" />

It should be a white circle, but it

[Coloryr]

<Style Selector="^:unchecked /template/ Border#NormalRectangle">
    <Setter Property="BorderBrush" Value="White" />
    <Setter Property="CornerRadius" Value="20" />
</Style>

it work on preview.