Update default style sheet with ::shadow selector

dot-atom/styles.less

@@ -12,10 +12,14 @@
 
 }
 
+// style the background and foreground colors on the atom-text-editor-element
+// itself
 atom-text-editor {
 
 }
 
-atom-text-editor .cursor {
+// To style other content in the text editor's shadow DOM, use the ::shadow
+// expression
+atom-text-editor::shadow .cursor {
 
 }