Functionality in general is not protected by copyright, although it may in some cases be protected by a patent. As the question noted, there are many IDEs and creating another one generally similar to those is not a violation of anyone's IP rights.
The "look and feel" of a piece of software has been held by US courts to be protected by copyright.
Broderbund Software. Inc., v. Unison World, Inc., 648 F. Supp. 1127 (N.D. Cal. 1986), was a relativcly early case in which the visual display elements of a software program were held to be protected by copyright. In the decision it is said that;
On the “Choose a Font” screen, no mechanical or practical factor compelled [defendant] to use those exact words (“Choose a Font”). He could have written: “Select a Font,” or “Indicate a Typeface Preference,” or “Which Type Style Do You Prefer,” or any combination of these terms. . . .The bottom line is that the designer of any program that performed the same functions as “Print Shop” had available a wide range of expression governed predominantly by artistic and not utilitarian considerations.
Another relevant early case was Digital Communications Associates, Inc. v. Softklone Distributing Corp., 659 F.Supp. 449 (N.D. Ga. 1987). In this case, the Defendant "oftklone intentionally set out to "clone" or copy the functionality and interface of the popular program Crosstalk XVI (I was a user of thwt program many years ago). The court wrote:
In the instant case, however, the arrange- ment of the status screen involves consider- able stylistic creativity and authorship above and beyond the ideas embodied in the status screen. It cannot be said that the idea of the status screen, i.e., using two symbol commands to change the operations of the computer program and reflecting that fact on a screen listing the computer program’s parameters/commands with their operative values, could not have been expressed in a large variety of ways. The defendants have never contended that they could not have arranged the parameters/commands in a wide variety of patterns without hampering the operation of their program.
and found copyright infringement on that basis.
Section 310 of Copyrightable Authorship:
What Can Be Registered by the US Copyright office, says:
The U.S. Copyright Office will not consider the so-called “look and feel” of a work. Invoking a work’s “feel” is not a viable substitute for an objective analysis of the work’s fixed and creative elements. See 4 MELVILLE & DAVID NIMMER, NIMMER ON Copyright §
13.03[A][c] (2013) (criticizing the use of “feel” as a “wholly amorphous referent” that “merely invites an abdication of analysis”).
The question asks:
How far can I go in copying the user interface before it becomes copyright infringement ?
There is no clear bright line on this matter. The more one copies interface elements and choices from a specific source, the stronger the case for infringement is. Using elements and choices that have become common industry practice in multiple works froim multiple developers and companies is probably safe. Clearly and extensively imitating the visual appearance and interface of a single specific work of software might well be infringement.
One would be wise to consult a lawyer with experience in copyright law, and specifically "look and feel" issues, beforign publishing such a work of software.
This issue is discussed extensively in Look And Feel In Computer Software (1993) by Jack Russo & Jamie Nafziger, published by Computerlaw Group LLP, which seems to be a law firm specializing in computer law.
The Law of Look and Feel by Peter Lee (Professor of Law and Chancellor’s Fellow, University of California, Davis.) & Madhavi Sunder (Senior Associate Dean for Academic Affairs and Martin Luther King, Jr. Professor of Law, University of California, Davis.) is a 60-page law review article dealing with this topic. There is far more detail in these publications (and many others to be found on the net) than I can summarize in an SE answer.
The question asks whether the use of a configurable interface which could, but does not by default, imitate an existing interface would be infringement.
I have not been able to find any case on point, or any discussion in a reliable source of this issue. Therefore, I am going to speculate.
If there is a single choice between overall "themes" or "shells" and one of them imitates the interface from a protected program without permission, that would probably be legally the same as if the program shipped with that interface. It might or might not be infringement, depending on the details, as discussed above.
If there are many settings, but the program is shipped with instructions advising a combination of settings that imitate another interface (or the developer or distributor provides such advice to users), that would probably be legally the same as if the program shipped in that configuration. If the developer provides a few preset configuration files which control these interface settings, and one such file results in settings that imitate another interface, that would also probably be legally the same as if the program shipped in that configuration.
If there are many settings and there is no particular guidance or advice to a user on what combination to use, nor any predefined settings file, but users must discover (if they choose) a group of settings which imitate another interface, that is probably not infringement.
I repeat that this section of the answer is based on general principals, but not on specific case law, nor on specific legal scholarship, and a court faced with this issue might rule otherwise. Before implementing this as a business plan, one might be wise to consult a lawyer with IP expertise.