use @base-background-color as a basis for the title-bar gradient to better accommodate themes that do not provide their own style for a title-bar

static/title-bar.less

@@ -23,6 +23,7 @@
     text-overflow: ellipsis;
   }
 
+  box-shadow: 0 1px 0 @title-bar-highlight inset;
   background-image: linear-gradient(@title-bar-gradient-focused);
   border-bottom: 1px solid @title-bar-border-color-focused;