The general "look and feel" of a website is not copyrightable. But too much similarity in the original, specific design aspects of another site may be infringement. It is not possible to express a rule that explains how much is too much similarity. Even if there is no bit by bit or "literal" copying, non-literal similarity can be infringement.
Trademarks cover brands and other source indicators. If a website style is distinctive enough that consumers actually associate it with one specific source or company, then it might acquire protection as "trade dress."
Please note: this answer is intended to inform and educate. It should not be taken as legal advice or a legal recommendation, as that would require a thorough review of all facts and circumstances. You may discuss the possibility of hiring me as a lawyer, but until we have an agreement in writing, there is no attorney-client relationship between us. Thank you, and good luck!
Perhaps, if the look & feel is distinctive enough to serve as a trademark or service mark, you might register it with the USPTO. Perhaps is the artwork is distinctive and original and non-functional you might have copyright protection that could be registered at the US Copyright Office. Perhaps if the design is functional but has a distinctive, nonobvious ornamental appearance you might get a design patent on it. Apple got a design patent on the arrangement of icons on the iPhone/iPad and obtained a judgment against Samsung for over a billion dollars (on appeal right now by Samsung) over several smartphones that knocked off that design.
Trademark protection and copyright protection are automatic but can be registered. Patent protection, such as design patents or utility patents, require an application to obtain protection.
Really now, you cannot do this on your own even with questions here. You need to get counsel from an IP attorney that you hire. Moreover, you need to do that so you can give details in confidence and so that such attorney can let you know if you have other legal issues you may not even recognize yet. Further you need that attorney to tell you strategically which forms of protection make the most sense for you in your special situation. If your Website is your business, you owe it to yourself to get this review so you can maximize your protection for your investment in your website.
I am not your lawyer and you are not my client. Free advice here is without recourse and any reliance thereupon is at your sole risk. This is done without compensation as a free public service. I am licensed in IL, MO, TX and I am a Reg. Pat. Atty. so advice in any other jurisdiction is strictly general advice and should be confirmed with an attorney licensed in that jurisdiction.
I agree with my colleagues. While there are some tactics you can employ, such as a design patent, there is really no formal way of protecting the "look and feel" of a website.
You should register any and all proprietary content with the US Copyright Office and of course conduct proper trademark due diligence and file an application with the USPTO.
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I agree with Attorney Morgan: If any protection is available for the non-functional elements of a website's look and feel, it is likely to be trade dress protection.
I blogged (briefly) about this topic more than two years ago. Please see the post at the link below.
This information does not constitute legal advice and does not establish an attorney-client relationship.