Improve Mac theme of the top of the nicklist.

im/themes/conversation.css

@@ -107,12 +107,15 @@ grippy {
 %ifdef XP_MACOSX
 .conv-nicklist-header {
   -moz-appearance: treeheadercell;
+  margin-right: -1px;
 }
 
 %endif
 .conv-nicklist-header-label {
 %ifdef XP_MACOSX
   -moz-margin-start: 3px !important;
+  margin-top: 1px !important;
+  margin-bottom: 0 !important;
 %else
   font-weight: bold;
   -moz-margin-start: 0 !important;