Bug 1887617 - follow-up, add missing colon. rs=bustage-fix DONTBUILD

--HG--
extra : amend_source : 2d2e3472673ace9e76ec34c15299f358358fd9f0

mail/themes/osx/mail/compose/messengercompose.css

@@ -18,7 +18,7 @@
     appearance: auto;
 
     #compose-toolbox {
-      appearance none;
+      appearance: none;
       background: transparent;
 
       /* This is conceptually a background, but putting this on a