messages: fix black page background when editing

changed the "body, .body {" to ".body" in the mail CSS so it doesn't
affect the web page. Seems to look fine still in gmail in both light and
dark modes. Fixes #446

mail/layout/header.mjml

@@ -8,7 +8,7 @@
             Color-scheme: light dark;
             supported-color-schemes: light dark;
         }
-        body, .body {
+        .body {
             background: #101010 !important;
             background-color: #101010 !important;
         }