Fix extensions dialog header color (#686)

theme/site/globals/site.variables

@@ -197,4 +197,10 @@
 
 @serialBackgroundColor: #fff;
 @serialGraphBackground: #F2F2F2;
-@serialConsoleBackground: @serialGraphBackground;
+@serialConsoleBackground: @serialGraphBackground;
+
+/*-------------------
+   Extensions
+--------------------*/
+
+@extensionsHeaderBackground: #999;