TestIcon.xaml

<UserControl x:Class="FluentTest.TestIcon"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
             xmlns:local="clr-namespace:FluentTest"
             xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
             Width="{Binding IconSize, RelativeSource={RelativeSource Mode=Self}}"
             Height="{Binding IconSize, RelativeSource={RelativeSource Mode=Self}}"
             d:DesignHeight="{Binding Tag, RelativeSource={RelativeSource Mode=Self}}"
             d:DesignWidth="{Binding Tag, RelativeSource={RelativeSource Mode=Self}}"
             BorderThickness="0"
             mc:Ignorable="d">
    <UserControl.Style>
        <Style TargetType="UserControl">
            <Setter Property="BorderBrush" Value="Black" />
            <Setter Property="BorderThickness" Value="0" />
            <Setter Property="Margin" Value="0" />
            <Setter Property="Padding" Value="1" />
            <Setter Property="Tag" Value="24" />
        </Style>
    </UserControl.Style>
    <Grid>
        <Border Background="{Binding IconBrush, RelativeSource={RelativeSource AncestorType=UserControl}}"
                BorderBrush="{Binding BorderBrush, RelativeSource={RelativeSource AncestorType=UserControl}}"
                BorderThickness="{Binding IconBorderThickness, RelativeSource={RelativeSource AncestorType=UserControl}}">
            <TextBlock HorizontalAlignment="Center"
                       VerticalAlignment="Center"
                       Text="{Binding IconSize, RelativeSource={RelativeSource AncestorType=UserControl}}" />
        </Border>
    </Grid>
</UserControl>