Update default color in App.css

The default color variable in App.css has been changed from #333333 to hotpink. This change affects all elements and components styled with this variable.

frontend/src/App.css

@@ -3,7 +3,7 @@
   --default-border-color: lightgrey;
   --default-border-radius: 15px;
   --header-height: 70px;
-  --default-color: #333333;
+  --default-color: hotpink;
   --primary-color: #6200ea;
   --error-color: red;
 }