If I change almost all the attributes of the "style.css" file and the supporting images such that the template looks totally different, then do I have to face any legal action?

Read the TOS and Rules that come with the file.

Jacob.

Nothing is written in the Readme file.

What is the template you are wishing to change? They all should come with the README file.
They are all different because there is a few different authors.

Jacob.

Generally if its not stated in any notice, it is implied by the designer that you cannot modify the layout and make it have the same design just different color. However, if you give it a complete make over, then you do not. Its always best to give some sort of credits to the original creator for their hard work.

I think it's okay to change all the template's aspects as long as you're not changing the copyright statement in the template's footer. Please correct me if I'm wrong, because I will plan to change my website template too.

I personally don't see a problem with changing things as long as you do leave the copyright in the footer. But make sure you always read the read-me file, because some authors say you cannot change any aspect of the template. Just my opinion, not official or anything.