Global style for GUI controls

You will have two DataGrids in your GUI, which share the style. Like CSS in HTML technologies, you can define styles in App.axaml:

...
    <Application.Styles>
        <FluentTheme />
        <StyleInclude Source="avares://Avalonia.Controls.DataGrid/Themes/Fluent.xaml" />
        <Style Selector="DataGrid">
            <Setter Property="GridLinesVisibility" Value="All" />
            <Setter Property="AutoGenerateColumns" Value="True" />
            <Setter Property="BorderThickness" Value="1" />
            <Setter Property="BorderBrush" Value="Gray" />
        </Style>
        <Style Selector="StackPanel > :is(Control)">
            <Setter Property="Margin" Value="5" />
        </Style>
    </Application.Styles>
...

Here we define global styles for DataGrids and each element in a StackPanel.