Avalonia/samples/ControlCatalog/MainView.xaml

<UserControl x:Class="ControlCatalog.MainView"
             xmlns="https://github.com/avaloniaui"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:controls="clr-namespace:ControlSamples;assembly=ControlSamples"
             xmlns:pages="clr-namespace:ControlCatalog.Pages"
             xmlns:models="clr-namespace:ControlCatalog.Models">
  <Grid>
    <Grid.Styles>
      
      <MediaStyles>
        <Style Selector="TextBlock.h2">
          <Setter Property="Foreground" Value="Red" />
        </Style>
      </MediaStyles>
      
      <Style Selector="TextBlock.h2">
        <Setter Property="TextWrapping" Value="Wrap" />
        <Setter Property="MaxWidth" Value="400" />
        <Setter Property="HorizontalAlignment" Value="Left" />
      </Style>
    </Grid.Styles>
    <TextBlock Classes="h2" Text="Testing" />
  </Grid>
</UserControl>