In my experience, there is only a few basic theoretical things:
1) correct drawings;
2) Ohm's law and
3) Kirghof's laws.
I myself learned this mostly from me father's electronics magazines when I was a boy.
The slightly more advanced staff includes some more math:
1) reactive impedance;
2) complex impedance;
UPD: 3) understanding in principle the work of transistors and an op amp,
but it all is in frame of the first ones. These I learned much more in past, after the university with the work.
And that's it. Actually there is of course some nisch staff, but you should learn it only if you actually work on it.
So, the 99.99% of real work here is developing intuitive apprehension. And you better prepare yourself, it will be hard as hell.
I think, what gives it is experience, reading datasheets, application notes, thinking a lot and UPD: watching EEVBlog.
I think it's relevant here, so I will mention that I'm an Electronics Design Engineer with almost 5 year experience, involved in measurement equipment developing.