Make background of checkmark icon in installer white.

Closes #3492079

project_template/web/profiles/drupal_cms_installer/theme/css/add-ons.css

@@ -53,7 +53,7 @@
   transform: translate(-50%, -50%); /* RTL */
   text-align: center;
   content: url('../images/check-circle.svg');
-  background-color: black;
+  background-color: white;
   color: white;
   border-radius: 50%;
 }