Basically my question is this: Is it acceptable style to mix Präteritum and Perfekt in a written sentence? For instance:
Obwohl er die Theorie verstanden hat, hatte er keine Ahnung von den praktischen Aspekten eines Vorstellungsgespräches.
I am preparing for my Telc B2 and want to avoid anything that might not be judged good German.
"Verstehen" gets a slightly different meaning if you set it to perfect tense.
"Er versteht die Theorie" (describing a status) und "Er hat die Theorie verstanden" are basically both statements about the present that mean the same thing. "Er hat die Theorie verstanden" means that he got a grasp on the theory at one point in the past, so now he knows it.
Hence, I would use Plusquamperfekt in that sentence to get the time order of things right. I think it's the same in English in this case (please correct me if I'm wrong):
Obwohl er die Theorie verstanden hatte, hatte er keine Ahnung von den praktischen Aspekten eines Vorstellungsgespräches.
Even though he had (not: has) understood the theory, he had no idea about the practical aspects of a job interview.
Präteritum / simple past doesn't have that meaning of verstehen, so you can also say (with pretty much the same meaning):
Obwohl er die Theorie verstand, hatte er keine Ahnung von den praktischen Aspekten eines Vorstellungsgespräches.
Even though he understood the theory, he had no idea about the practical aspects of a job interview.
What @HalvarF said is absolutely correct, i'd like to add some general remarks:
You can mix tenses to express Vorzeitigkeit or Nachzeitigkeit. Your example is wrong for that matter, because the sentence describes two things (understanding the theory, understanding the practice) that happen at the same time - and for that you should use the same tense.
Example:
Ich ging, nachdem ich mich verabschiedet hatte.
Ich gehe, nachdem ich mich verabschiedet habe.
Depending on the main sentence ("i go") being in Präsens or Präteritum what happened before (greeting) is in Perfekt or Plusquamperfekt.
Things which happen at the same time (or in no particular order) are related in the same time. Notice that storytelling (Erzählzeit) happens in Präteritum:
Ich wuchs auf, ging zur Schule, traf meine erste Liebe und bekam Pickel.
Everything happens more or less at the same time (or at least not caused by each other) and hence every Verb is in the same time (Präteritum, because this is the style of relating a story).
Ich wuchs auf, ging zur Schule und traf meine erste Liebe, nachdem ich Pickel bekommen hatte.
One thing takes place clearly before the other. What happened before (getting acne) is in Plusquamperfect, what happened later is in Präteritum still, because it is the continuation of the storytelling.
Notice, that this concept is necessary only in-sentence. This:
Ich wuchs auf, ging zur Schule und traf meine erste Liebe. Davor bekam ich Pickel.
is perfectly OK, because of the separated sentences.
