macos: sync NSAppearance for app based on window-theme setting

macos/Sources/Ghostty/AppState.swift

@@ -71,6 +71,16 @@ extension Ghostty {
             _ = ghostty_config_get(config, &v, key, UInt(key.count))
             return v;
         }
+        
+        /// The window theme as a string.
+        var windowTheme: String? {
+            guard let config = self.config else { return nil }
+            var v: UnsafePointer<Int8>? = nil
+            let key = "window-theme"
+            guard ghostty_config_get(config, &v, key, UInt(key.count)) else { return nil }
+            guard let ptr = v else { return nil }
+            return String(cString: ptr)
+        }
 
         init() {
             // Initialize ghostty global state. This happens once per process.