Bug 1811988 - Follow-up: Set the #singleMessage background-color on Mac. r=aleca

Differential Revision: https://phabricator.services.mozilla.com/D168137

--HG--
extra : moz-landing-system : lando

mail/themes/linux/mail/messageHeader.css

@@ -9,8 +9,7 @@
 @import url("chrome://messenger/skin/shared/messageHeader.css");
 
 /* ::::: for the entire area ::::: */
-#singleMessage,
-#calendar-task-details-container {
+#singleMessage {
   color: WindowText;
   background-color: AppWorkspace;
 }

mail/themes/osx/mail/messageHeader.css

@@ -8,6 +8,10 @@
 
 @import url("chrome://messenger/skin/shared/messageHeader.css");
 
+#singleMessage {
+  background-color: -moz-Dialog;
+}
+
 /* ::::: msg header toolbars ::::: */
 
 .inline-toolbox {

mail/themes/windows/mail/messageHeader.css

@@ -9,8 +9,7 @@
 @import url("chrome://messenger/skin/shared/messageHeader.css");
 
 /* ::::: for the entire area ::::: */
-#singleMessage,
-#calendar-task-details-container {
+#singleMessage {
   color: WindowText;
   background-color: ButtonFace;
 }