Minor CSS tweaks (#227)

docs/extra.css

@@ -22,3 +22,12 @@ input {
         height: 48px;
     }
 }
+
+.text-center {
+    text-align: center !important;
+}
+
+/* needed because images have `max-width: 100%` in the theme */
+img {
+    height: auto;
+}