Merge pull request #2645 from unoplatform/dev/doti/docs-themeservice-…

…registration

docs: Adjust registration specifics for `ThemeService`

doc/Learn/ThemeService/HowTo-UseThemeService.md

@@ -1,40 +1,29 @@
 ---
 uid: Uno.Extensions.ThemeService.Overview
 ---
-
 # How to use Theme Service
 
 This topic explains how to use the `ThemeService` for runtime theme switching and persisting user theme preferences.
 
-## Step-by-step
+## Automatic Registration with DI
 
-1. **Register ThemeService**:
-    Add the `ThemeService` to your project's host builder configuration.
+> [!NOTE]
+> The `ThemeService` is automatically registered when you enable DI (Dependency Injection) or Extensions UnoFeature in your Uno Platform project. This means you typically *do not* need to explicitly register it using `UseThemeSwitching` unless you are not using DI.
 
-    ```csharp
-    public partial class App : Application
-    {
-        protected override void OnLaunched(LaunchActivatedEventArgs args)
-        {
-            var builder = this.CreateBuilder(args)
-                .Configure(host => host
-                    .UseThemeSwitching()
-                );
-        }
-    }
-    ```
+## Step-by-step (Typical Usage with DI)
 
-2. **Consume ThemeService**:
-    Inject the ThemeService into your view models or other services where you need to manipulate the theme.
+1. **Consume ThemeService**: Inject the `ThemeService` into your view models or other services where you need to manipulate the theme.
 
     ```csharp
     public class SettingsViewModel
     {
         private readonly IThemeService _themeService;
+
         public SettingsViewModel(IThemeService themeService)
         {
             _themeService = themeService;
         }
+
         public async Task ToggleThemeAsync()
         {
             var currentTheme = _themeService.Theme;
@@ -44,6 +33,29 @@ This topic explains how to use the `ThemeService` for runtime theme switching an
     }
     ```
 
+## Step-by-step (Manual Registration without DI or Advanced Scenarios)
+
+If you are *not* using DI, Extensions, or require more control over the registration process, you can manually register the `ThemeService`:
+
+1. When using the Uno.Sdk, follow this guide on how to add `ThemeService` [UnoFeature](xref:Uno.Features.Uno.Sdk#managing-the-unosdk-version).
+
+1. **Register ThemeService**: Add the `ThemeService` to your project's host builder configuration using `UseThemeSwitching`.
+
+    ```csharp
+    public partial class App : Application
+    {
+        protected override void OnLaunched(LaunchActivatedEventArgs args)
+        {
+            var builder = this.CreateBuilder(args)
+                .Configure(host => host
+                    .UseThemeSwitching()
+                );
+        }
+    }
+    ```
+
+1. **Consume ThemeService:** (Same as the DI usage) Inject the `ThemeService` as shown in the previous example.
+
 ## Source Code
 
 [ThemeService Implementation](https://github.com/unoplatform/uno.extensions/blob/51c9c1ef14f686363f946588733faecc5a1863ff/src/Uno.Extensions.Core.UI/Toolkit/ThemeService.cs)