customization - What changes we need to make to a theme so it can be installed as a MU Theme? in WordPress

You really need to specify how it doest work. There are multisite-specific theme tags. 99% of the themes out there for WordPress work in a network.

If you're getting errors, check your error logs.

If it'st grabbing image th8umbnails, that's a known issue -t with multisite or the timthumb script, but how the theme itself calls the images.

http://www.binarymoon.co.uk/2009/10/timthumb-wordpress-mu/

Edit: actually, I have tested the Irresistible theme in mu. Worked for me. A quick checked reveals it's looking for images in a subfolder that ist there.