FIX: Background color for settings overriden filter (#30221)

This was fixed previously but must have regressed, we
are showing a darker grey background around the
"Only show overridden" checkbox for our Settings tab
in config pages.

app/assets/stylesheets/common/admin/admin_config_area.scss

@@ -90,6 +90,7 @@
 
   &__settings {
     .admin-site-settings-filter-controls {
+      background: var(--primary-very-low);
       margin-bottom: 1em;
     }
 

app/assets/stylesheets/common/admin/site-settings.scss

@@ -11,6 +11,7 @@
 .admin-plugin-config-area {
   &__settings {
     .admin-site-settings-filter-controls {
+      background: var(--primary-very-low);
       margin-bottom: 1em;
     }