styling changes for #275

Here are the styling changes moved to c3.css regarding #275

c3.css

@@ -66,6 +66,11 @@
 .c3-text {
 }
 
+.c3-text.c3-empty {
+  fill: #808080;
+  font-size: 2em;
+}
+
 /*-- Line --*/
 
 .c3-line {
@@ -188,4 +193,4 @@
 .c3-chart-arc .c3-gauge-value {
   fill: #000;
   font-size: 28px;
-}
+}