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

You really need to specify how it does not work. There are no 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’s not grabbing image th8umbnails, that’s a known issue – not 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 is not there.

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