avaloniaProject/AvaloniaApplication5/App.axaml

<Application xmlns="https://github.com/avaloniaui"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             x:Class="AvaloniaApplication5.App"
             xmlns:local="using:AvaloniaApplication5"
             RequestedThemeVariant="Default">
             <!-- "Default" ThemeVariant follows system theme variant. "Dark" or "Light" are other available options. -->

    <Application.DataTemplates>
        <local:ViewLocator/>
    </Application.DataTemplates>
  
    <Application.Styles>
        <FluentTheme />
		<Style Selector="TextBlock.AuthTBStyle">
			<Setter Property="FontSize" Value="24"/>
			<Setter Property="FontWeight" Value="Bold"/>
		</Style>
		<Style Selector="TextBlock.ErrorMessage">
			<Setter Property="FontSize" Value="16"/>
			<Setter Property="FontWeight" Value="Bold"/>
			<Setter Property="FontFamily" Value="Arial"/>
			<Setter Property="FontStyle" Value="Italic"/>
			<Setter Property="Foreground" Value="Red"/>
		</Style>
		
		
		
    </Application.Styles>
</Application>