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="Dark">

    <Application.Styles>
        <FluentTheme />
        <StyleInclude Source="Themes/IndustrialStyles.axaml"/>
    </Application.Styles>

    <Application.Resources>
        <ResourceDictionary>
            <ResourceDictionary.MergedDictionaries>
                <ResourceInclude Source="Themes/IndustrialTheme.axaml"/>
            </ResourceDictionary.MergedDictionaries>
        </ResourceDictionary>
    </Application.Resources>

</Application>