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

<ResourceDictionary xmlns="https://github.com/avaloniaui"
                    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
                    xmlns:converters="clr-namespace:Avalonia.Controls.Converters;assembly=Avalonia.Controls">
  <Design.PreviewWith>
    <Border Padding="20">
      <StackPanel Spacing="10">
        <ProgressBar VerticalAlignment="Center" IsIndeterminate="True" />
        <ProgressBar VerticalAlignment="Center" Value="5" Maximum="10" />
        <ProgressBar VerticalAlignment="Center" Value="50" />
        <ProgressBar VerticalAlignment="Center" Value="50" Minimum="25" Maximum="75" />
        <ProgressBar HorizontalAlignment="Left" IsIndeterminate="True" Orientation="Vertical" />
      </StackPanel>
    </Border>
  </Design.PreviewWith>

  <converters:StringFormatConverter x:Key="StringFormatConverter" />

  <ControlTheme x:Key="{x:Type ProgressBar}"
                TargetType="ProgressBar">
    <Setter Property="Background" Value="{DynamicResource ThemeAccentBrush4}" />
    <Setter Property="Foreground" Value="{DynamicResource ThemeAccentBrush}" />
    <Setter Property="Template">
      <ControlTemplate TargetType="ProgressBar">
        <Grid>
          <Border Background="{TemplateBinding Background}"
                  BorderBrush="{TemplateBinding BorderBrush}"
                  BorderThickness="{TemplateBinding BorderThickness}"
                  CornerRadius="{TemplateBinding CornerRadius}">
            <Panel>
              <Border Name="PART_Indicator"
                      Background="{TemplateBinding Foreground}"
                      IsVisible="{Binding !IsIndeterminate, RelativeSource={RelativeSource TemplatedParent}}" />
              <Border Name="PART_IndeterminateIndicator"
                      Background="{TemplateBinding Foreground}"
                      IsVisible="{Binding IsIndeterminate, RelativeSource={RelativeSource TemplatedParent}}" />
            </Panel>
          </Border>
          <LayoutTransformControl Name="PART_LayoutTransformControl"
                                  HorizontalAlignment="Center"
                                  VerticalAlignment="Center"
                                  IsVisible="{Binding ShowProgressText, RelativeSource={RelativeSource TemplatedParent}}">
            <TextBlock Foreground="{DynamicResource ThemeForegroundBrush}">
              <TextBlock.Text>
                <MultiBinding Converter="{StaticResource StringFormatConverter}">
                  <TemplateBinding Property="ProgressTextFormat" />
                  <Binding Path="Value"
                           RelativeSource="{RelativeSource TemplatedParent}" />
                  <TemplateBinding Property="Percentage" />
                  <TemplateBinding Property="Minimum" />
                  <TemplateBinding Property="Maximum" />
                </MultiBinding>
              </TextBlock.Text>
            </TextBlock>
          </LayoutTransformControl>
        </Grid>
      </ControlTemplate>
    </Setter>

    <Style Selector="^:horizontal /template/ Border#PART_Indicator">
      <Setter Property="HorizontalAlignment" Value="Left" />
      <Setter Property="VerticalAlignment" Value="Stretch" />
    </Style>
    <Style Selector="^:vertical /template/ Border#PART_Indicator">
      <Setter Property="HorizontalAlignment" Value="Stretch" />
      <Setter Property="VerticalAlignment" Value="Bottom" />
    </Style>
    <Style Selector="^:horizontal">
      <Setter Property="MinWidth" Value="200" />
      <Setter Property="MinHeight" Value="16" />
    </Style>
    <Style Selector="^:vertical">
      <Setter Property="MinWidth" Value="16" />
      <Setter Property="MinHeight" Value="200" />
    </Style>
    <Style Selector="^:vertical /template/ LayoutTransformControl#PART_LayoutTransformControl">
      <Setter Property="LayoutTransform">
        <Setter.Value>
          <RotateTransform Angle="90" />
        </Setter.Value>
      </Setter>
    </Style>

    <Style Selector="^:horizontal:indeterminate /template/ Border#PART_IndeterminateIndicator">
      <Style.Animations>
        <Animation Easing="LinearEasing"
                   IterationCount="Infinite"
                   Duration="0:0:3">
          <KeyFrame Cue="0%">
            <Setter Property="TranslateTransform.X" Value="{Binding IndeterminateStartingOffset, RelativeSource={RelativeSource TemplatedParent}}" />
          </KeyFrame>
          <KeyFrame Cue="100%">
            <Setter Property="TranslateTransform.X" Value="{Binding IndeterminateEndingOffset, RelativeSource={RelativeSource TemplatedParent}}" />
          </KeyFrame>
        </Animation>
      </Style.Animations>
      <Setter Property="Width" Value="{Binding TemplateProperties.ContainerWidth, RelativeSource={RelativeSource TemplatedParent}}" />
    </Style>
    <Style Selector="^:vertical:indeterminate /template/ Border#PART_IndeterminateIndicator">
      <Style.Animations>
        <Animation Easing="LinearEasing"
                   IterationCount="Infinite"
                   Duration="0:0:3">
          <KeyFrame Cue="0%">
            <Setter Property="TranslateTransform.Y" Value="{Binding IndeterminateStartingOffset, RelativeSource={RelativeSource TemplatedParent}}" />
          </KeyFrame>
          <KeyFrame Cue="100%">
            <Setter Property="TranslateTransform.Y" Value="{Binding IndeterminateEndingOffset, RelativeSource={RelativeSource TemplatedParent}}" />
          </KeyFrame>
        </Animation>
      </Style.Animations>
      <Setter Property="Height" Value="{Binding TemplateProperties.ContainerWidth, RelativeSource={RelativeSource TemplatedParent}}" />
    </Style>
  </ControlTheme>
</ResourceDictionary>