[PLUGINS][PinnedNotes] Replacing arbitary size values with common variables

public/plugins/PinnedNotes/assets/css/pinned-notes.css

@@ -8,6 +8,6 @@
     opacity: 1 !important;
 }
 .feed.pinned {
-    margin-top: 2em;
-    margin-bottom: 6em;
+    margin-top: var(--m);
+    margin-bottom: var(--m);
 }