Hey Friends I found this gr8 plugin that can be used to put categories in tag cloud and it would still use the same Tag cloud with same styling which you see while using one-theme. It would just put all of your categories in it.
Here is the link Il Filosofo » A Plugin for WordPress 2.3: Include Categories in the Tag Cloud
I am not using the Category cloud on my site. Because it aint solving my purpose.
But this could be helpful for others.
Cheers