App.axaml

<Application xmlns="https://github.com/avaloniaui"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:local="clr-namespace:CustomTitleBarTemplate"
             x:Class="CustomTitleBarTemplate.App">
    <Application.DataTemplates>
        <local:ViewLocator/>
    </Application.DataTemplates>

    <Application.Styles>
      <StyleInclude Source="avares://Avalonia.Themes.Default/DefaultTheme.xaml"></StyleInclude>
      <StyleInclude Source="avares://Avalonia.Themes.Fluent/FluentLight.xaml"></StyleInclude>
    </Application.Styles>

    <Application.Resources>
      <Brush x:Key="MacOsWindowTitleColor">#4D4D4D</Brush>
      <Brush x:Key="MacOsTitleBarBackground">#DEE1E6</Brush>
    </Application.Resources>
</Application>