Add "Follow OS theme" option

src/Ryujinx/App.axaml.cs

@@ -84,7 +84,7 @@ namespace Ryujinx.Ava
             ApplyConfiguredTheme();
         }
 
-        private void ApplyConfiguredTheme()
+        public void ApplyConfiguredTheme()
         {
             try
             {
@@ -92,13 +92,16 @@ namespace Ryujinx.Ava
 
                 if (string.IsNullOrWhiteSpace(baseStyle))
                 {
-                    ConfigurationState.Instance.UI.BaseStyle.Value = "Dark";
+                    ConfigurationState.Instance.UI.BaseStyle.Value = "Auto";
 
                     baseStyle = ConfigurationState.Instance.UI.BaseStyle;
                 }
 
+                ThemeVariant systemTheme = DetectSystemTheme();
+
                 RequestedThemeVariant = baseStyle switch
                 {
+                    "Auto" => systemTheme,
                     "Light" => ThemeVariant.Light,
                     "Dark" => ThemeVariant.Dark,
                     _ => ThemeVariant.Default,
@@ -111,5 +114,29 @@ namespace Ryujinx.Ava
                 ShowRestartDialog();
             }
         }
+
+        // Convert Avalonia.Platform.PlatformThemeVariant to the expected ThemeVariant type
+        private Avalonia.Styling.ThemeVariant ConvertThemeVariant(Avalonia.Platform.PlatformThemeVariant platformThemeVariant)
+        {
+            switch (platformThemeVariant)
+            {
+                case Avalonia.Platform.PlatformThemeVariant.Dark:
+                    return Avalonia.Styling.ThemeVariant.Dark;
+
+                case Avalonia.Platform.PlatformThemeVariant.Light:
+                    return Avalonia.Styling.ThemeVariant.Light;
+
+                default:
+                    return Avalonia.Styling.ThemeVariant.Default;
+            }
+        }
+
+        private ThemeVariant DetectSystemTheme()
+        {
+            var platformSettings = this.PlatformSettings;
+            var colorValues = platformSettings.GetColorValues();
+
+            return ConvertThemeVariant(colorValues.ThemeVariant);
+        }
     }
 }

src/Ryujinx/Assets/Locales/en_US.json

@@ -298,6 +298,7 @@
   "GameListContextMenuToggleFavorite": "Toggle Favorite",
   "GameListContextMenuToggleFavoriteToolTip": "Toggle Favorite status of Game",
   "SettingsTabGeneralTheme": "Theme:",
+  "SettingsTabGeneralThemeAuto": "Follow OS theme",
   "SettingsTabGeneralThemeDark": "Dark",
   "SettingsTabGeneralThemeLight": "Light",
   "ControllerSettingsConfigureGeneral": "Configure",

src/Ryujinx/UI/ViewModels/SettingsViewModel.cs

@@ -405,7 +405,13 @@ namespace Ryujinx.Ava.UI.ViewModels
             GameDirectories.Clear();
             GameDirectories.AddRange(config.UI.GameDirs.Value);
 
-            BaseStyleIndex = config.UI.BaseStyle == "Light" ? 0 : 1;
+            BaseStyleIndex = config.UI.BaseStyle.Value switch
+            {
+                "Auto" => 0,
+                "Light" => 1,
+                "Dark" => 2,
+                _ => 0
+            };
 
             // Input
             EnableDockedMode = config.System.EnableDockedMode;
@@ -492,7 +498,13 @@ namespace Ryujinx.Ava.UI.ViewModels
                 config.UI.GameDirs.Value = gameDirs;
             }
 
-            config.UI.BaseStyle.Value = BaseStyleIndex == 0 ? "Light" : "Dark";
+            config.UI.BaseStyle.Value = BaseStyleIndex switch
+            {
+                0 => "Auto",
+                1 => "Light",
+                2 => "Dark",
+                _ => "Auto"
+            };
 
             // Input
             config.System.EnableDockedMode.Value = EnableDockedMode;

src/Ryujinx/UI/Views/Settings/SettingsUIView.axaml

@@ -62,6 +62,9 @@
                         <ComboBox SelectedIndex="{Binding BaseStyleIndex}"
                                   HorizontalContentAlignment="Left"
                                   MinWidth="100">
+                            <ComboBoxItem>
+                                <TextBlock Text="{locale:Locale SettingsTabGeneralThemeAuto}" />
+                            </ComboBoxItem>
                             <ComboBoxItem>
                                 <TextBlock Text="{locale:Locale SettingsTabGeneralThemeLight}" />
                             </ComboBoxItem>

src/Ryujinx/UI/Windows/MainWindow.axaml.cs

@@ -2,6 +2,7 @@ using Avalonia;
 using Avalonia.Controls;
 using Avalonia.Controls.Primitives;
 using Avalonia.Interactivity;
+using Avalonia.Platform;
 using Avalonia.Threading;
 using FluentAvalonia.UI.Controls;
 using Ryujinx.Ava.Common;
@@ -87,6 +88,25 @@ namespace Ryujinx.Ava.UI.Windows
             }
         }
 
+        // Event handler for detecting OS theme change when using "Follow OS theme" option
+        private void OnPlatformColorValuesChanged(object? sender, PlatformColorValues e)
+        {
+            if (Application.Current is App app)
+            {
+                app.ApplyConfiguredTheme();
+            }
+        }
+
+        protected override void OnClosed(EventArgs e)
+        {
+            base.OnClosed(e);
+            if (this.PlatformSettings != null)
+            {
+                // Unsubscribe to the ColorValuesChanged event
+                this.PlatformSettings.ColorValuesChanged -= OnPlatformColorValuesChanged;
+            }
+        }
+
         protected override void OnApplyTemplate(TemplateAppliedEventArgs e)
         {
             base.OnApplyTemplate(e);
@@ -370,6 +390,9 @@ namespace Ryujinx.Ava.UI.Windows
 
             Initialize();
 
+            // Subscribe to the ColorValuesChanged event
+            this.PlatformSettings.ColorValuesChanged += OnPlatformColorValuesChanged;
+
             ViewModel.Initialize(
                 ContentManager,
                 StorageProvider,