To disable dark mode support in .NET MAUI, you need to use these code snippets in each platform

Android

In the MainApplication add the following line:

public MainApplication(IntPtr handle, JniHandleOwnership ownership) : base(handle, ownership)
{
     AppCompatDelegate.DefaultNightMode = AppCompatDelegate.ModeNightNo;
}

iOS

In the info.plist file, add the following keyvaluepair:

    <key>UIUserInterfaceStyle</key>
    <string>Light</string>

WinUI

In the App.xaml.cs (the windows version one) add the following line:

public App()
	{
		this.InitializeComponent();
        RequestedTheme = ApplicationTheme.Light;
	}