Hi Sanket
TodoMVC now has a SAPUI5 example, this is a very good resource showing how to add custom style to you SAPUI5 application.
http://todomvc.com/labs/architecture-examples/sapui5/
Below is the code for the view
the code uses various ways to change the style, i think you are after something like
oLabel.addStyleClass('MyCustomClass');
Cheers
John P