Just to let you know that I live in GMT + 5:45 timezone so unfortunately when you’re expecting the reply I was in the bed.
While changing only CSS class it should not make the site blank ? Can you check if only CSS class is changed or mistakenly you changed other code too ?
Try to enable the debug of WordPress and see if that help you or not . Set define(‘WP_DEBUG’, true); on wp-config.php . This link might help you :
Let us know how it goes.