Hi rajshekar,
I have never done that kind of customization, but roughly if you are using Alfresco Share 3.x I believe you will have to customize some resources the header web script (alfresco/site-webscripts/org/alfresco/components/header) using the web-extension directory. On the other hand, if you are on a 4.x version perhaps would be possible to do it with the help of a extension module with the appropriate evaluator.
Let me know if you need further details.
Regards.