[FIX] mass_mailing: restore background color option for mailing snippets

Changing the background color was not working
because 'o_mail_snippet_general' having a background color white.
while bg-* colors are applied on a parent of 'o_mail_snippet_general'

closes odoo/odoo#42513

X-original-commit: 751dd4c16f8e6c9e373bd9be3a783cde2833947d
Signed-off-by: Quentin Smetz (qsm) <qsm@odoo.com>

addons/mass_mailing/static/src/scss/themes/theme_default.scss

@@ -72,7 +72,6 @@ td:not([align]) {
     width: 100%;
     max-width: $o-mm-def-body-width; // should not be necessary thanks to mail wrapper
     margin: 0 auto;
-    background-color: $o-mm-def-body-bg;
 
     h1, h2, h3, h4, h5, h6, .h1, .h2, .h3, .h4, .h5, .h6 {
         color: $o-mm-def-text-color;