[FIX] mass_mailing: properly background-color .o_mail_snippet_general

The "o_mail_snippet_general" class implies a background color but when
coloring the snippet that color was applied to its parent. This ensures
that the color is applied to .o_mail_snippet_general instead.

closes odoo#71500

Related: odoo/enterprise#18619
Signed-off-by: David Monjoie (dmo) <dmo@odoo.com>

Author: Zynton

addons/mass_mailing/views/snippets_themes.xml

@@ -865,7 +865,10 @@
         data-selector=".note-editable > div:not(.o_layout), .note-editable .oe_structure > div, td, th"
         data-exclude=".o_mail_no_resize, .o_mail_no_options"/>
 
-    <div data-selector=".note-editable > div:not(.o_layout), .note-editable .oe_structure > div, td, td.o_mail_no_colorpicker div:first-child, th"
+    <div data-selector=".note-editable > div:not(.o_layout),
+        .note-editable .oe_structure > div:not(:has(> .o_mail_snippet_general)),
+        .note-editable .oe_structure > div > .o_mail_snippet_general,
+        td, td.o_mail_no_colorpicker div:first-child, th"
          data-exclude=".o_mail_no_colorpicker, .o_mail_no_options">
         <we-colorpicker string="Background Color"
             data-select-style="true"