Zanim zrobi się ciemno, chcę wrócić do domu.

Zanim means before when it introduces a clause:

• Zanim zrobi się ciemno = Before it gets dark

By contrast, przed is a preposition, so it goes with a noun phrase, not a full clause:

• przed zmrokiem = before dusk
• przed kolacją = before dinner

So a simple rule is:

• zanim + clause
• przed + noun / noun phrase

Because zanim zrobi się ciemno is a subordinate clause, and in Polish subordinate clauses are normally separated by a comma.

So both of these need a comma:

• Zanim zrobi się ciemno, chcę wrócić do domu.
• Chcę wrócić do domu, zanim zrobi się ciemno.

This is more regular in Polish than in English, where comma use can be looser.

Formally, zrobi się looks like a present-tense form, but because zrobić is a perfective verb, this form refers to the future.

So:

• robi się = is becoming / gets
• zrobi się = will become / will get

That is why zrobi się ciemno means it will get dark.

This is completely normal in Polish: perfective verbs do not have a true present meaning.

Here się is part of the expression zrobić się, which often means to become or to get.

So:

• zrobić się zimno = to get cold
• zrobić się cicho = to become quiet
• zrobić się ciemno = to get dark

It is not really reflexive in the English sense. You do not need to translate it word for word. It is simply part of how this verb phrase works in Polish.

Because Polish uses ciemno in this kind of impersonal expression.

• ciemny is an adjective, for example ciemny pokój = a dark room
• ciemno is used in expressions like it is dark or it gets dark

So:

• Jest ciemno = It is dark
• Robi się ciemno = It is getting dark
• Zrobi się ciemno = It will get dark

There is no noun here for an adjective to agree with, so ciemny would not fit.

Polish usually does not use a dummy subject like English it in weather, time, and general-condition expressions.

English says:

• It is dark
• It is raining
• It is late

Polish usually just says:

• Jest ciemno
• Pada
• Jest późno

So in zrobi się ciemno, the idea of English it is understood, but Polish does not need to say it.

After chcieć = to want, Polish normally uses an infinitive for the second verb.

So:

• chcę wrócić = I want to return
• chcę iść = I want to go
• chcę spać = I want to sleep

This is similar to English want to + verb.

So chcę wrócić is the standard structure.

This is an aspect question.

• wrócić is perfective: to return, to get back, as a completed event
• wracać is imperfective: to be returning, to return repeatedly, or to focus on the process

In this sentence, the speaker wants to complete one return home before darkness falls, so wrócić is the natural choice.

Compare:

• Chcę wrócić do domu. = I want to get back home.
• Lubię wracać do domu pieszo. = I like walking home / returning home on foot.

The preposition do takes the genitive case, and dom in the genitive singular is domu.

So:

• dom = nominative
• do domu = genitive after do

This phrase is very common:

• iść do domu = to go home
• wrócić do domu = to return home

Even though English often says simply home, Polish usually uses do domu in this kind of phrase.

Yes. A very natural alternative is:

• Chcę wrócić do domu, zanim zrobi się ciemno.

Both word orders are correct.

The difference is mainly one of emphasis:

• Zanim zrobi się ciemno, chcę wrócić do domu.
  This foregrounds the time limit first.
• Chcę wrócić do domu, zanim zrobi się ciemno.
  This starts with the main point: I want to go home.

Yes, but ja is usually unnecessary unless you want emphasis or contrast.

Because chcę already tells you the subject is I, Polish often leaves ja out.

So:

• Chcę wrócić do domu. = neutral, normal
• Ja chcę wrócić do domu. = emphatic, contrastive, like I want to go home

For example, you might use ja if you are contrasting yourself with someone else.

Yes, zanim będzie ciemno is possible, but it has a slightly different feel.

• zanim zrobi się ciemno = before it gets dark
  Focus on the change
• zanim będzie ciemno = before it is dark
  Focus on the state

In many situations, both work, but zrobi się ciemno is very natural when talking about darkness falling.

Another common alternative is:

• Zanim się ściemni, chcę wrócić do domu.

This is also very natural and means essentially the same thing.

Yes, it is natural and correct.

A native speaker would understand it immediately, and it sounds normal. Depending on style, you might also hear:

• Chcę wrócić do domu, zanim zrobi się ciemno.
• Zanim się ściemni, chcę wrócić do domu.

But the original sentence is absolutely fine.