[IMP] mass_mailing: add extra data to themeParams for custom styling

By adding the `style` attribute to the `.o_layout` element
it can be customized according to the data collected
with `themeParams`, making it possible to add a custom background color
in our template design, for example.

See b7537e1d66b4ca26e12f4be16eb1c30632ecee61

closes odoo/odoo#83701

Signed-off-by: Antoine Guenet <age@odoo.com>

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

@@ -269,6 +269,7 @@ var MassMailingFieldHtml = FieldHtml.extend({
         }
         var $newLayout = $('<div/>', {
             class: 'o_layout oe_unremovable oe_unmovable bg-200 ' + themeParams.className,
+            style: themeParams.layoutStyles,
             'data-name': 'Mailing',
         }).append($new_wrapper);
 
@@ -418,7 +419,8 @@ var MassMailingFieldHtml = FieldHtml.extend({
                         return imagesInfo[filename];
                     }
                     return imagesInfo.all;
-                }
+                },
+                layoutStyles: $theme.data('layout-styles'),
             };
         });
         $themes.parent().remove();