[IMP] mass_mailing: apply default background color to all HTML mailings

This applies a default background color to all HTML mailings in order to
emphasize the separation between the mailing itself and its body.

task-2554899

X-original-commit: 75cbf5a559f891546c92f9079eb4d8dd22ac5e6b
Part-of: odoo/odoo#77724

addons/mass_mailing/static/src/js/mass_mailing_widget.js

@@ -288,7 +288,7 @@ var MassMailingFieldHtml = FieldHtml.extend({
             $new_wrapper.append($('<div class="row"/>').append($newWrapperContent));
         }
         var $newLayout = $('<div/>', {
-            class: 'o_layout oe_unremovable oe_unmovable ' + themeParams.className,
+            class: 'o_layout oe_unremovable oe_unmovable bg-200 ' + themeParams.className,
             'data-name': 'Mailing',
         }).append($new_wrapper);