Applying custom CSS to indexLabel is not available as of now, but you can customize toolTip either by using contentFormatter or by over-riding toolTip class as shown in this jsfiddle.
In bar/column charts, indexLabels are placed inside or outside based on the available space. You can set indexLabelPlacement in dataSeries level to outside so that indexLabels are shown outside the bar.
Thanks for your reply. Is there any way to extend the functionality of charts from outside like updating prototype to enhance the functionality so it would be very helpful because sometime we need to add functionality which is not available for now.