theme switching & add AI logo to theme

Frontend/lib/main.dart

@@ -43,6 +43,10 @@ class _MzanziInnovationHubState extends State<MzanziInnovationHub> {
         setState(() {
           theme.mode = "Light";
         });
+      } else if (_themeMode == ThemeMode.dark) {
+        setState(() {
+          theme.mode = "Dark";
+        });
       } else {
         setState(() {
           theme.mode = "Dark";

Frontend/lib/mih_theme/mih_theme.dart

@@ -227,6 +227,18 @@ class MyTheme {
     return loading;
   }
 
+  AssetImage aiLogoImage() {
+    if (mode == "Dark") {
+      return const AssetImage(
+        'images/mzansi_ai-dark.png',
+      );
+    } else {
+      return const AssetImage(
+        'images/mzansi_ai-light.png',
+      );
+    }
+  }
+
   void setScreenType(double width) {
     if (width <= 800) {
       screenType = "mobile";