is this illegal?

By taking an open source project and striping the style sheet out and then modifying it for your own works to then go on and distribute it... is that illegal?

This is basicly the rule for OPEN SOURCE released under the GNU GPL public licence...

In a few sentences, what is the GPL?
The GPL license gives end-users additional rights that are normally reserved to the copyright holder, such as distributing and making copies of work , make derivative works (ie add new features ), and the freedom to sell the work to others. These additional rights are granted as long as the end-user also grants these same rights to their end-users under the terms of the GPL, and as long as the individual abides by the terms of the GPL.

hmm, maybe it's time for my new browser release...Bozilla Firebox.

It depends. Will your product be open-source? If so, then it is perfectly legal. Otherwise, according to the GNU GPL, it is not. That is the main condition.

I'm not so sure about that one Goalie:

"and the freedom to sell the work to others."
so as long as he distributes it under the GPL he can sell it... That's what it looks like from my view anyways.

Yes, the GPL requires the script to be open-spurce. Believe it or not, open source does NOT mean free. It means the source may be redistributed in any way, but it doesn't mean that they can't charge for it.

If you change the source to so much of an extent, how would one find out of your doings?

this isnt my works however i compared two style sheets of two different forums and they were identical apart from the little specifc bits that were removed. The style sheet was subsequently use to make a new piece of software to rival that of the one the style sheet was taken from

Is the style sheet listed as an open source artifact? If someone's done work, but haven't released it under GPL, then you could be looking for trouble, Especially if you're going into direct competition. However the stylesheet is in the public domain as its served with each visit to the site - a tough call.
--Ax

Nothing goes into the public domain unless you deliberately designate it as such. You cannot lose your copyright by putting it on the Internet or using it on a Web site. Every country that is a signatory to the Berne Convention (basically, all first-world countries, and a lot of third-world ones too) enforces a copyright-by-default system. That is, EVERY little bit of EVERYTHING you write is copyrighted by default. You don't have to do anything. If you can prove to be the originator of the work (by registering it or by other means), courts will come down in your favor.

And as a general comment, I'd just note that Open Source isn't public domain. In fact, it relies on copyright to help with enforcement. If the original author of the stylesheet can prove he/she did it, and if it can be proved that it itself wasn't cobbled together from other previously-existing examples, then that author can enforce whatever rules he/she wants.

Personally, while I think copyright is a very strong tool, I also think that there is no way a stylesheet can be unique enough to differentiate it from the millions of others out there. Maybe, it could be used to show a judge that there are similarities enough to warrant a subpoena, seizure, or search. Then the code for the competing site could be captured legally, and inspected, and if shown to have great similarity, lawyers could party all the way to the bank. If lawyers party.

-Tony