How Can I make Themes For N-Gage please

You can't. The N-Gage is based on Series 60 1.2 and Themes support appeared first in Series 60 2.0 and the Nokia 6600 phone.

The N-Gage cannot be upgraded to a newer Series 60 release.

Your best bet to get something almost similar is to buy an application like "Smart Launcher":
http://www.allaboutsymbian.com/shop/