Good afternoon, I would suggest the following option:

Set the BackgroundColor property in the AppShell.xaml file:

<?xml version="1.0" encoding="UTF-8" ?>
<Shell
    x:Class="MauiAppTestTheme.AppShell"
    xmlns="http://schemas.microsoft.com/dotnet/2021/maui"
    xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
    xmlns:local="clr-namespace:MauiAppTestTheme"
    Title="MauiAppTestTheme"
    **Shell.BackgroundColor="{AppThemeBinding Light={StaticResource Black}, Dark=  {StaticResource Black}}">**

    <ShellContent
        Title="Home"
        ContentTemplate="{DataTemplate local:MainPage}"
        Route="MainPage" />

</Shell>

This will allow you to always use the dark theme on every page of the app by default