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.

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

@@ -0,0 +1,8 @@
+namespace AIStudio.Settings.DataModel;
+
+public enum Themes
+{
+    SYSTEM = 0,
+    LIGHT,
+    DARK,
+}

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

@@ -0,0 +1,16 @@
+namespace AIStudio.Settings.DataModel;
+
+public static class ThemesExtensions
+{
+    public static string GetName(this Themes theme)
+    {
+        return theme switch
+        {
+            Themes.SYSTEM => "Synchronized with the operating system settings",
+            Themes.LIGHT => "Always use light theme",
+            Themes.DARK => "Always use dark theme",
+            
+            _ => "Unknown setting",
+        };
+    }
+}