misc: chore: change ThemeManager ThemeChanged to a basic Action since both arguments are unused

src/Ryujinx/Common/ThemeManager.cs

@@ -4,11 +4,11 @@ namespace Ryujinx.Ava.Common
 {
     public static class ThemeManager
     {
-        public static event EventHandler ThemeChanged;
+        public static event Action ThemeChanged;
 
         public static void OnThemeChanged()
         {
-            ThemeChanged?.Invoke(null, EventArgs.Empty);
+            ThemeChanged?.Invoke();
         }
     }
 }

src/Ryujinx/UI/ViewModels/AboutWindowViewModel.cs

@@ -27,7 +27,7 @@ namespace Ryujinx.Ava.UI.ViewModels
             ThemeManager.ThemeChanged += ThemeManager_ThemeChanged;
         }
 
-        private void ThemeManager_ThemeChanged(object sender, EventArgs e)
+        private void ThemeManager_ThemeChanged()
         {
             Dispatcher.UIThread.Post(() => UpdateLogoTheme(ConfigurationState.Instance.UI.BaseStyle.Value));
         }