Should I always force the use of active voice in my dissertation? I decided to use first person ("we") and active voice as the primary style of writing my dissertation in the domain of Computer Science. I have read some discussions about this topic and the people generally agree that we have to be consistent.

Nonetheless I also noticed that are some instances in which the passive voice seems to be a better fit, like when I want to emphasize the action rather than the actor or when I don't know who is the actor (or it doesn't matter).

It seems hard to me in those situations to force the use of active voice (maybe because English is not my first language). To what degree should I force the use of active voice? Are there some guidelines regarding this topic?
The goal of style guides is to give suggestions for how to write well. These are guidelines and suggestions, not immutable rules. Once you understand what the base suggestions are, and why they’re there, you can begin to understand when not to apply them. As you said, if you don’t know who the “actor” is, an active-voice sentence becomes more difficult to make clear without sounding stilted.

(I’ve tried writing a paper entirely in active voice when I was still a student. I wasn’t particularly thrilled with the results.)