❔ WPF Border TemplateBinding

            <Style x:Key="RoundBorder">
                <Style.Resources>
                    <Style TargetType="Border">
                        <Setter Property="CornerRadius" Value="3" />
                    </Style>
                </Style.Resources>
            </Style>

public static class ThemeProperties {
    public static float GetRoundedCornerRadius(DependencyObject obj) {
        return (float)obj.GetValue(RoundedCornerRadiusProperty);
    }
    public static void SetRoundedCornerRadius(DependencyObject obj, float value) {
        obj.SetValue(RoundedCornerRadiusProperty, value);
    }
    public static readonly DependencyProperty RoundedCornerRadiusProperty =
        DependencyProperty.RegisterAttached("RoundedCornerRadius", typeof(float), typeof(ThemeProperties), new PropertyMetadata(3f));
}