YZWater3/src/YZWater.Avalonia/App.axaml

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

    <Application.Styles>
        <FluentTheme />
    </Application.Styles>

</Application>