Reading Tae Kim's guide, I've seen the phrase:
Where (I think) the term 誰か is being used as topic for the verb 知る.
Is that grammatically correct ?
I don't know 誰か is used as topic but 誰か知りませんか? is common and I think that is grammatically correct.
It is translated as "Does anyone know ～?" and your sentence is "Does anyone know who stole?".
It's because it's the most straightforward choice.
Using は would sound as if you are wondering if there's at least one person who knows it among people present there, and using が would sound as if you are wondering if there's a person in the world who knows it.
Incidentally, the topic of the whole sentence is 誰が盗んだのか, (edit) that's why it's not marked with を.
Both が and か could be used after 誰. However their functions are different as follows:
が indicates 'sentence subject'. "誰が盗んだのか" literally translates to "About who stole (something)".
か also indicates 'sentence subject'. However, it has connotation of uncertainty and doubt. "誰か知りませんか" literally translates to "(I am not sure whether somebody could find out who stole it), but does anybody know?"
誰か来たようだ I am not sure who came, but I think somebody came.
むこうに誰かいる I am not sure who there is, but I think somebody is there.