Added color themes (dark & light) (#148)

app/MindWork AI Studio/Settings/DataModel/DataApp.cs

@@ -2,6 +2,11 @@ namespace AIStudio.Settings.DataModel;
 
 public sealed class DataApp
 {
+    /// <summary>
+    /// The preferred theme to use.
+    /// </summary>
+    public Themes PreferredTheme { get; set; } = Themes.SYSTEM;
+    
     /// <summary>
     /// Should we save energy? When true, we will update content streamed
     /// from the server, i.e., AI, less frequently.