Learn Hardcore Croatian: Večeras prikazuju dokumentarni film. - They are screening a documentary film tonight.

Croatian often leaves subject pronouns out because the verb ending already shows who is doing the action. The form prikazuju is 3rd person plural, so it already means they show / they are showing. You would add oni only for emphasis or contrast.

Who does they refer to here?

Usually it means an unspecified group such as a TV station, a cinema, a festival, or the people in charge of the program. Croatian often uses 3rd person plural this way when the exact subject is not important. In English, this can also be translated with an indefinite they, or sometimes with a passive idea such as a documentary is being shown tonight.

Why is the verb in the present tense if the action happens tonight?

Croatian often uses the present tense for planned, scheduled, or near-future events, especially when there is a time word like večeras. So prikazuju can refer to something happening later today. English does something similar in sentences like They’re showing it tonight.

What exactly does večeras mean?

Večeras means tonight or this evening. It is an adverb, so it does not need a preposition. In this sentence, it sets the time for the whole event.