updating block name to class and removing grey modifier after it was added to the body tag

opentech/public/standardpages/templates/standardpages/index_page.html

 {% extends "base.html" %}
 {% load wagtailcore_tags wagtailimages_tags navigation_tags static %}
-{% block main_modifier %}wrapper--light-grey-bg wrapper--bottom-space{% endblock %}
 {% block body_class %}light-grey-bg{% endblock %}
+{% block main_class %}wrapper--bottom-space{% endblock %}
 {% block content %}
 <div class="wrapper">
     <section class="section section--main">

opentech/templates/base.html

@@ -143,7 +143,7 @@
             <svg class="header__icon header__icon--pixels header__icon--pixels-right"><use xlink:href="#hero-standard-right-pixels"></use></svg>
         </header>
 
-        <main class="wrapper wrapper--large wrapper--main {% block main_modifier %}{% endblock %}">
+        <main class="wrapper wrapper--large wrapper--main {% block main_class %}{% endblock %}">
             {% block content %}{% endblock %}
         </main>