Documentation/misc/wpf/styling.md

Styling

The most obvious difference from other XAML frameworks is that Avalonia uses a CSS-like styling system. Styles aren't stored in a Resources collection as in WPF, they are stored in a separate Styles collection:

<UserControl>
    <UserControl.Styles>
        <!-- Make TextBlocks with the h1 style class have a font size of 24 points -->
        <Style Selector="TextBlock.h1">
            <Setter Property="FontSize" Value="24"/>
        </Style>
    </UserControl.Styles>
    <TextBlock Classes="h1">Header</TextBlock>
<UserControl>