month-view: grey out invalid days

It should be very clear that days outside the current
month are actually not valid. The current drawing
algorithm does not differentiate that, thought, and
draws everything at the same color.

Fix that by greying out the days outside the current
month.

https://bugzilla.gnome.org/show_bug.cgi?id=763771

data/theme/gtk-styles.css

@@ -56,6 +56,10 @@ calendar-view.lines {
     color: alpha(@theme_fg_color, 0.4);
 }
 
+calendar-view.offset {
+    background-color: alpha(@theme_bg_color, 0.5);
+}
+
 /* Year View CSS */
 .year-view {
     padding: 0px;