Avalonia/src/Avalonia.Themes.Simple/Controls/NumericUpDown.xaml

<ResourceDictionary xmlns="https://github.com/avaloniaui"
                    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
                    x:ClassModifier="internal">
  <ControlTheme x:Key="{x:Type NumericUpDown}"
                TargetType="NumericUpDown">
    <Setter Property="Background" Value="{DynamicResource ThemeBackgroundBrush}" />
    <Setter Property="BorderBrush" Value="{DynamicResource ThemeBorderMidBrush}" />
    <Setter Property="BorderThickness" Value="{DynamicResource ThemeBorderThickness}" />
    <Setter Property="Padding" Value="4" />
    <Setter Property="Template">
      <ControlTemplate>
        <DataValidationErrors>
          <ButtonSpinner Name="PART_Spinner"
                         HorizontalContentAlignment="Stretch"
                         VerticalContentAlignment="Stretch"
                         IsTabStop="False"
                         AllowSpin="{TemplateBinding AllowSpin}"
                         Background="{TemplateBinding Background}"
                         BorderBrush="{TemplateBinding BorderBrush}"
                         BorderThickness="{TemplateBinding BorderThickness}"
                         ButtonSpinnerLocation="{TemplateBinding ButtonSpinnerLocation}"
                         CornerRadius="{TemplateBinding CornerRadius}"
                         ShowButtonSpinner="{TemplateBinding ShowButtonSpinner}">
            <TextBox Name="PART_TextBox"
                     MinWidth="20"
                     Margin="4"
                     Padding="{TemplateBinding Padding}"
                     HorizontalContentAlignment="{TemplateBinding HorizontalContentAlignment}"
                     VerticalContentAlignment="{TemplateBinding VerticalContentAlignment}"
                     AcceptsReturn="False"
                     Background="{TemplateBinding Background}"
                     BorderThickness="0"
                     IsReadOnly="{TemplateBinding IsReadOnly}"
                     Text="{TemplateBinding Text}"
                     TextAlignment="{TemplateBinding TextAlignment}"
                     TextWrapping="NoWrap"
                     Watermark="{TemplateBinding Watermark}"
                     InnerLeftContent="{Binding InnerLeftContent, RelativeSource={RelativeSource TemplatedParent}}"
                     InnerRightContent="{Binding InnerRightContent, RelativeSource={RelativeSource TemplatedParent}}"/>
          </ButtonSpinner>
        </DataValidationErrors>
      </ControlTemplate>
    </Setter>
  </ControlTheme>
</ResourceDictionary>