Added XSS exercises

static/css/style.css

@@ -1,12 +1,10 @@
 body {
     background-color: #e8eef7;
+    padding: 2em;
 }
 
-form {
-    display: block;
-    margin: 0.5em auto 2.5em;
+#quiz {
     padding: 1em;
-    width: 45em;
     background-color: white;
     border: 1px solid #ccc;
 }
@@ -19,9 +17,15 @@ form {
     font-weight: bold;
 }
 
+.center-block {
+    display: block;
+    margin-left: auto;
+    margin-right: auto;
+}
+
 textarea {
     width: 75%;
-    height: 7em;
+    height: 9em;
 }
 
 input {