The big problem with telling you where the line is, is that nobody really knows. The remedies for licence violations are legal ones, so the terms of a licence mean what a court says they do - and I am not aware of any legal authority to date laying out a clear set of rules for when one work is a derivative of another.
Part of the problem is that copyleft is fairly new. Copying was first restricted in 1662, by restrictions on presses introduced by the Licensing of the Press Act, and a formal copyright system first came into being under the Statute of Anne in 1710. So we've had about three hundred years of precedent (both binding and advisory), legislative clarification, harmonisation, international treaties, and so on, to the point where, although the balance of power between creator, publisher and consumer isn't entirely a happy one, it is fairly well-understood.
But the GPL was first published in 1989. So although we have three hundred years of courts clarifying how to use copyright to stop people copying, we have less than a tenth of that history of the courts clarifying how to use copyright to help people copy, provided that they share likewise.
One of the areas where the water is still very murky is a bright-line test for a work being derivative of another, and it is the engagement of derivation that enables the control of a subsequent work via copyright, thus in turn engaging the GPL. Until courts and legislatures start to lay down bright-line tests, or failing that, until precedent starts to accrete around the matter, noone really knows.
If you take someone else's code, and make changes to it, provided those changes aren't de minimis, I don't know anyone who would argue that the result isn't a derivative work. I think, as you say, that it is generally agreed that binary linking and shared address space constitute derivation, but this is not universally agreed. As apsillers kindly points out, "mere aggregation" is thought by nearly no-one to involve the creation of a derivative work. The kind of interaction you ask about - Wordpress themes - are even less clear-cut, and I don't understand Wordpress well enough to begin to know where to put it on that sliding scale. But the thrust of my answer is that since there is no binding agreement on the stops and data points on that scale, it would be pointless to do so. Sorry.