msfs-popout-panel-manager/MainApp/CustomControl/Themes/NumericUpDown.xaml

<ResourceDictionary
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:local="clr-namespace:MSFSPopoutPanelManager.MainApp.CustomControl">
    <local:FontSizeConverter x:Key="FontSizeConverter" />
    <Style TargetType="{x:Type local:NumericUpDown}">
        <Setter Property="Template">
            <Setter.Value>
                <ControlTemplate TargetType="{x:Type local:NumericUpDown}">
                    <Border
                        Background="{TemplateBinding Background}"
                        BorderBrush="{TemplateBinding BorderBrush}"
                        BorderThickness="{TemplateBinding BorderThickness}">
                        <Grid>
                            <Grid.ColumnDefinitions>
                                <ColumnDefinition />
                                <ColumnDefinition Width="Auto" />
                            </Grid.ColumnDefinitions>
                            <TextBox
                                x:Name="PART_TextBox"
                                Margin="-20,0,0,0"
                                HorizontalAlignment="Center"
                                VerticalAlignment="Center"
                                BorderThickness="0" />
                            <Grid Grid.Column="1">
                                <RepeatButton
                                    x:Name="PART_ButtonUp"
                                    Width="20"
                                    Height="20"
                                    Margin="-40,0,0,0"
                                    Padding="2,0"
                                    Content="▲"
                                    FontSize="{Binding FontSize, Converter={StaticResource FontSizeConverter}, RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type local:NumericUpDown}}}" />
                                <RepeatButton
                                    x:Name="PART_ButtonDown"
                                    Width="20"
                                    Height="20"
                                    Margin="1,0,0,0"
                                    Padding="2,0"
                                    Content="▼"
                                    FontSize="{Binding FontSize, Converter={StaticResource FontSizeConverter}, RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type local:NumericUpDown}}}" />
                            </Grid>
                        </Grid>
                    </Border>
                </ControlTemplate>
            </Setter.Value>
        </Setter>
        <Setter Property="BorderBrush" Value="Gray" />
        <Setter Property="BorderThickness" Value="1" />
        <Setter Property="Width" Value="100" />
    </Style>
</ResourceDictionary>