Add the "Auto" theme option in setting (#6611)

* Add "Follow OS theme" option

* Update App.axaml.cs

* Add "Follow OS theme" option

* Update App.axaml.cs

* Remove `this`

* Remove annotation for nullable reference

* Change into switch expression to make it concise

* Change comments to XML docs

* Update en_US.json

* Fix icons in About dialog do not response to "auto" theme

The theme icons seemingly use Dark variant event when the OS theme is light. In addition, I added ThemeManager common to make it accessible for both App and AboutWindow

* Newline at the end

* newline moment

* Update ThemeManager.cs

* bait to switch to lf

* change to lf

* temp. revert

* Add back ThemeManager.cs common, pls pass the format check

* I found the mistake: should have put `ThemeManager.OnThemeChanged();` in try block

Finally solve the formatting check

* test formatting

* Update App.axaml.cs

* Ok i seem to forget to add version lol

* Fix info CA1816

src/Ryujinx/Common/ThemeManager.cs

@@ -0,0 +1,14 @@
+using System;
+
+namespace Ryujinx.Ava.Common
+{
+    public static class ThemeManager
+    {
+        public static event EventHandler ThemeChanged;
+
+        public static void OnThemeChanged()
+        {
+            ThemeChanged?.Invoke(null, EventArgs.Empty);
+        }
+    }
+}