Avalonia/AvaloniaApplication3/App.axaml

<Application xmlns="https://github.com/avaloniaui"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             x:Class="AvaloniaApplication3.App"
             xmlns:local="using:AvaloniaApplication3"
             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="Button.blueBtn">
			<Setter Property="Background" Value="Blue"></Setter>
			<Setter Property="Foreground" Value="White"></Setter>
		</Style>
		<Style Selector="TextBlock">
			<Setter Property="Foreground" Value="Black"></Setter>
			<Setter Property="FontSize" Value="16"/>
		</Style>
		<Style Selector="TextBlock.title">
			<Setter Property="Foreground" Value="Black"></Setter>
			<Setter Property="FontSize" Value="30"/>
			<Setter Property="FontWeight" Value="SemiBold"/>
		</Style>
		<Style Selector="TextBlock.error">
			<Setter Property="Foreground" Value="Red"></Setter>
			<Setter Property="FontWeight" Value="SemiBold"/>
		</Style>
		<Style Selector="StackPanel.mainWinImgPanel">
			<Setter Property="Background" Value="#E2F0FF"/>
		</Style>
    </Application.Styles>
</Application>