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

<ResourceDictionary xmlns="https://github.com/avaloniaui"
                    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
                    x:ClassModifier="internal">
  <Design.PreviewWith>
    <Border Padding="20">
      <StackPanel Spacing="20">
        <NumericUpDown Minimum="0"
                       Maximum="10"
                       Increment="0.5"
                       Watermark="Enter text" />
        <NumericUpDown Minimum="0"
                       Maximum="10"
                       Increment="0.5"
                       VerticalContentAlignment="Center"
                       HorizontalContentAlignment="Center"
                       ButtonSpinnerLocation="Left"
                       Watermark="Enter text" />
      </StackPanel>
    </Border>
  </Design.PreviewWith>

  <ControlTheme x:Key="{x:Type NumericUpDown}" TargetType="NumericUpDown">
    <Setter Property="Foreground" Value="{DynamicResource TextControlForeground}" />
    <Setter Property="Background" Value="{DynamicResource TextControlBackground}" />
    <Setter Property="BorderThickness" Value="{DynamicResource TextControlBorderThemeThickness}" />
    <Setter Property="BorderBrush" Value="{DynamicResource TextControlBorderBrush}" />
    <Setter Property="MinHeight" Value="{DynamicResource TextControlThemeMinHeight}" />
    <Setter Property="MinWidth" Value="{DynamicResource TextControlThemeMinWidth}" />
    <Setter Property="Padding" Value="{DynamicResource TextControlThemePadding}" />
    <Setter Property="CornerRadius" Value="{DynamicResource ControlCornerRadius}" />
    <Setter Property="Template">
      <ControlTemplate>
        <ButtonSpinner Name="PART_Spinner"
                       Background="{TemplateBinding Background}"
                       BorderThickness="{TemplateBinding BorderThickness}"
                       BorderBrush="{TemplateBinding BorderBrush}"
                       CornerRadius="{TemplateBinding CornerRadius}"
                       Padding="0"
                       MinWidth="0"
                       HorizontalContentAlignment="Stretch"
                       VerticalContentAlignment="Stretch"
                       AllowSpin="{TemplateBinding AllowSpin}"
                       ShowButtonSpinner="{TemplateBinding ShowButtonSpinner}"
                       DataValidationErrors.Errors="{TemplateBinding (DataValidationErrors.Errors)}"
                       ButtonSpinnerLocation="{TemplateBinding ButtonSpinnerLocation}">
          <TextBox Name="PART_TextBox"
                   Background="Transparent"
                   BorderBrush="Transparent"
                   Margin="-1"
                   Padding="{TemplateBinding Padding}"
                   MinWidth="0"
                   Foreground="{TemplateBinding Foreground}"
                   Watermark="{TemplateBinding Watermark}"
                   IsReadOnly="{TemplateBinding IsReadOnly}"
                   VerticalContentAlignment="{TemplateBinding VerticalContentAlignment}"
                   HorizontalContentAlignment="{TemplateBinding HorizontalContentAlignment}"
                   Text="{TemplateBinding Text}"
                   TextAlignment="{TemplateBinding TextAlignment}"
                   AcceptsReturn="False"
                   TextWrapping="NoWrap" />
        </ButtonSpinner>
      </ControlTemplate>
    </Setter>
  </ControlTheme>
</ResourceDictionary>