Add NEWs bullet for theme update

Author: jiho

NEWS.md

@@ -1,5 +1,9 @@
+
 # ggplot2 2.1.0.9000 
 
+* Themes are more homogeneous visually, and match `theme_grey` better.
+  (@jiho, #1679)
+
 * `position_stack()` and `position_fill()` now sorts the stacking order so it 
   matches the order of the grouping. Use level reordering to alter the stacking 
   order. The default legend and stacking order is now also in line. The default