Disclaimer: I'm not a lawyer, and this is not qualified legal advice. Use at your own risk.
Your first step is to figure out which license the website itself is under. It may be different from the software; or there may be no explicit license for the website (HTML, CSS, JS etc) itself.
Nota bene: The above is valid if static or dynamic linking is the criteria for determining whether your own code is a derived work or not. Opinions differ. If the codebase is licensed under the GPL version 3, the terms are stricter and your own code will be more likely to also fall under the GPL version 3. See this nice overview of the GPL version 2 and 3.
My personal advice: Either stay away from GPL licensed software, or pay the original author for a copy of the work under another, closed-source license. The original author can always license the work to you under another license, and in fact many will do so for a modest payment. Using GPL code for commercial uses is tricky; it's IMHO easier to either keep away from GPL code or obtain it under a non-copyleft license.
Website design and copy is copyrighted, which is separate from the licensing stuff of the software in question.
So no, you cannot steal it.
However if you ask they might very well say "Sure, take it!" So I'd ask!