Migration of Drupal 6 to Drupal 7 I have a Drupal 6 site with different content types, and node references. I also have a custom theme. I want to migrate to Drupal 7. 
I use the Node export, and Node import modules, but they are not working perfectly, as I am not able to import node references.

Is there any easy way to do this?
Read the UPGRADE.txt
Have a look at the CCK Module for D7. It comes with a migration submodule.
The them has to be adapted to D7 manually. Have a look at this for reference.