As far as my knowledge goes, technically, everything is copyrighted or has sort of copyright attachment to it even if they are allowing it under a permissive license such as GPL, CC, etc...
As for the key CSS styling goes, I (I'm not a lawyer, but just my thoughts) don't think it will be an issue if you write your own custom CSS. You CSS code should look different and that means the output rendered will look different too. So no issue there.
But if you want to use SE's code, then I would recommend reading all of the information(including sub-links within) given in Jeff's post. I would also suggest you reading the Stack Exchange Network Terms of Service. Look for any "Exclusive Copyright" or "All Rights Reserved" related stuff. If exists, then you must adhere those terms and conditions. Also, if a copyright issue occurs, usually the original copyright holder i.e. Stack Exchange, Inc. will send a DMCA takedown notice to you.
For additional references, even though they not exactly related to the CSS styling copyright, I would suggest reading the answer (and comments too) for Who owns the copyright to S[OFU] content?. It's worth and contains lots of good information related to Copyright material. Also, here is what Jeff Atwood says (as for the contributed content goes): Source: Attribution Required
All the content contributed to Stack Overflow or other Stack Exchange
sites is cc-wiki (aka cc-by-sa) licensed, intended to be shared and
remixed. We even provide all our data as a convenient data dump,
seeded by us.
But our cc-wiki licensing, while intentionally permissive, does