Add WindowSkin: let a separate plugin theme the settings window

ConfigurationManager.Shared/ConfigurationManager.cs

@@ -50,6 +50,25 @@ public class ConfigurationManager : BaseUnityPlugin
         private bool _focusSearchBox;
         private string _searchString = string.Empty;
 
+        /// <summary>
+        /// Skin to draw the settings window with, for example to match the look of the game. Null (the default) keeps the default look.
+        /// The window, its controls and tooltips are drawn with this skin, and its window style draws the window background.
+        /// The dimmed backdrop behind a window that has not been moved yet keeps the default skin.
+        /// Plugin and category headers use the custom styles named <see cref="PluginHeaderStyleName"/> and <see cref="CategoryHeaderStyleName"/>
+        /// if the skin has them.
+        /// </summary>
+        public GUISkin WindowSkin { get; set; }
+
+        /// <summary>
+        /// Name of the custom style in <see cref="WindowSkin"/> used for plugin headers, if present.
+        /// </summary>
+        public const string PluginHeaderStyleName = "ConfigurationManager.PluginHeader";
+
+        /// <summary>
+        /// Name of the custom style in <see cref="WindowSkin"/> used for category headers, if present.
+        /// </summary>
+        public const string CategoryHeaderStyleName = "ConfigurationManager.CategoryHeader";
+
         /// <summary>
         /// Event fired every time the manager window is shown or hidden.
         /// </summary>
@@ -309,10 +328,15 @@ private void OnGUI()
                     if (GUI.Button(_screenRect, string.Empty, GUI.skin.box) && !SettingWindowRect.Contains(mousePosition))
                         DisplayingWindow = false;
 
-                    ImguiUtils.DrawWindowBackground(SettingWindowRect);
+                    // A custom skin draws the window background with its own window style
+                    if (WindowSkin == null)
+                        ImguiUtils.DrawWindowBackground(SettingWindowRect);
                 }
 
+                var defaultSkin = GUI.skin;
+                if (WindowSkin != null) GUI.skin = WindowSkin;
                 var newRect = GUILayout.Window(WindowId, SettingWindowRect, (GUI.WindowFunction)SettingsWindow, "Plugin / mod settings");
+                GUI.skin = defaultSkin;
 
                 if (newRect != SettingWindowRect)
                 {

ConfigurationManager.Shared/SettingFieldDrawer.cs

@@ -99,32 +99,41 @@ public static void DrawCenteredLabel(string text, params GUILayoutOption[] optio
             GUILayout.EndHorizontal();
         }
 
-        private static GUIStyle _categoryHeaderSkin;
-        public static void DrawCategoryHeader(string text)
+        /// <summary>
+        /// Header style of the current skin: its custom style with the given name if it has one, otherwise a centered copy of its label.
+        /// Rebuilt only when the skin changes, so the default look builds it once as before.
+        /// </summary>
+        private static GUIStyle GetHeaderStyle(string customStyleName, int fontSize, ref GUIStyle style, ref GUISkin styleSkin)
         {
-            if (_categoryHeaderSkin == null)
+            var skin = GUI.skin;
+            if (style == null || styleSkin != skin)
             {
-                _categoryHeaderSkin = GUI.skin.label.CreateCopy();
-                _categoryHeaderSkin.alignment = TextAnchor.UpperCenter;
-                _categoryHeaderSkin.wordWrap = true;
-                _categoryHeaderSkin.stretchWidth = true;
-                _categoryHeaderSkin.fontSize = 14;
+                style = skin.FindStyle(customStyleName);
+                if (style == null)
+                {
+                    style = skin.label.CreateCopy();
+                    style.alignment = TextAnchor.UpperCenter;
+                    style.wordWrap = true;
+                    style.stretchWidth = true;
+                    style.fontSize = fontSize;
+                }
+                styleSkin = skin;
             }
+            return style;
+        }
 
-            GUILayout.Label(text, _categoryHeaderSkin);
+        private static GUIStyle _categoryHeaderSkin;
+        private static GUISkin _categoryHeaderSkinFor;
+        public static void DrawCategoryHeader(string text)
+        {
+            GUILayout.Label(text, GetHeaderStyle(ConfigurationManager.CategoryHeaderStyleName, 14, ref _categoryHeaderSkin, ref _categoryHeaderSkinFor));
         }
 
         private static GUIStyle _pluginHeaderSkin;
+        private static GUISkin _pluginHeaderSkinFor;
         public static bool DrawPluginHeader(GUIContent content, bool isCollapsed)
         {
-            if (_pluginHeaderSkin == null)
-            {
-                _pluginHeaderSkin = GUI.skin.label.CreateCopy();
-                _pluginHeaderSkin.alignment = TextAnchor.UpperCenter;
-                _pluginHeaderSkin.wordWrap = true;
-                _pluginHeaderSkin.stretchWidth = true;
-                _pluginHeaderSkin.fontSize = 15;
-            }
+            GetHeaderStyle(ConfigurationManager.PluginHeaderStyleName, 15, ref _pluginHeaderSkin, ref _pluginHeaderSkinFor);
 
             if (isCollapsed) content.text += "\n...";
             return GUILayout.Button(content, _pluginHeaderSkin, GUILayout.ExpandWidth(true));