Add.xaml

<ResourceDictionary xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
    <DrawingBrush x:Key="AddSub" Stretch="Uniform">
        <DrawingBrush.Drawing>
            <DrawingGroup>
                <DrawingGroup.Children>
                    <GeometryDrawing Brush="{DynamicResource UiElementForeground}" Geometry="F1 M 39,46L 46,46L 46,39L 51,39L 51,46L 58,46L 58,51L 51,51L 51,58L 46,58L 46,51L 39,51L 39,46 Z M 31,25L 38,25L 38,18L 43,18L 43,25L 50,25L 50,30L 43,30L 43,37L 38,37L 38,30L 31,30L 31,25 Z M 18,39L 25,39L 25,32L 30,32L 30,39L 37,39L 37,44L 30,44L 30,51L 25,51L 25,44L 18,44L 18,39 Z "/>
                </DrawingGroup.Children>
            </DrawingGroup>
        </DrawingBrush.Drawing>
    </DrawingBrush>
</ResourceDictionary>