Kali Linux, a Linux distribution for ethical hackers and penetration testers, has an "Undercover" mode that makes its UI look like Windows 10. It copies the desktop background, Windows logo Start menu icon, and the overall look and feel of the UI. The purpose is to allow ethical hackers/penetration testers to use Kali Linux in public places (e.g. a client's office) without drawing attention from bystanders who might not know that the testing is authorized. (The default Kali UI and [especially] desktop background are quite distinctive, and would likely look somewhat suspicious even to someone who doesn't know about Kali. Someone who does know about it would almost certainly be concerned to see someone using it at their workplace.) Assuming Offensive Security (the company that develops Kali Linux) didn't get permission from Microsoft, is this legal? I doubt Microsoft would give a license for an intentionally deceptive clone of Windows's UI.
Copyright would generally protect all aspects of the Windows operating system. Therefore, to the extend that Kali is visually similar to Windows, it might be illegal infringement. It would not be infringement if those elements were used with permission from MS (note that the product is available on the Microsoft store). Since you semi-stipulate that they do not have permission, we can move to the possibility of a fair use defense. That defense might be successful since at least apparently there would be no effect on market and the copying is somewhat transformative.
Finally, the finder of fact would have to decide if those elements of Kali are "substantially similar" to the Windows originals. It could be found that the similarity which you perceive is due to "copying the idea", not copying the expression. E.g. the idea of a manila folder as an icon is not protected, so it becomes a fairly technical discussion centering on copyright law and "look and feel".
Separate from the copyright concerns discussed by user67726, there are also trademark concerns. The Lanham Act (15 U.S.C. § 1051) forbids using someone else's trademark in to deceive or cause confusion. It's hard to argue that an "undercover" mode doesn't deceive.
Of course, this assumes that the undercover mode makes use of a Microsoft trademark. This could be the case for the start menu icon, since it's part of Microsoft's logo.