Finish exercise backgrounds1.html

Exercise from https://developer.mozilla.org/en-US/docs/Learn/CSS/Building_blocks/Test_your_skills_backgrounds_and_borders

Author: mtdowner

learn/tasks/backgrounds/backgrounds1.html

@@ -16,11 +16,14 @@
 
     <style class="editable">
       .box {
-  
+        border: 3px solid #000;
+        border-radius: 10px;
+        background-image: url("balloons.jpg");
       }
-
       h2 {
-        
+        color: #fff;
+        background-color: rgba(0,0,0,0.5);
+        text-shadow: 3px 3px #000;
       }
     </style>
   </head>