Learn Hardcore Finnish: Keittiö on vielä pimeä, joten sytytän lampun. - The kitchen is still dark, so I turn on the lamp.

Finnish has no articles like the or a/an. Whether something is “the kitchen” or “a kitchen” is usually understood from context. Here, Keittiö simply means kitchen, and the situation makes it clear which kitchen is meant.

Why is it Keittiö on vielä pimeä and not something like Keittiössä on vielä pimeää?

Both are possible, but they mean slightly different things:

Keittiö on vielä pimeä = “The kitchen is still dark.” (The kitchen as a whole is described as dark; pimeä is a predicate adjective.)
Keittiössä on vielä pimeää = “It’s still dark in the kitchen.” (Focus is on the state/amount of darkness in that location; pimeää is partitive and more like “darkness”.)

So your sentence is the straightforward “X is dark” structure: [noun] + on + [adjective].

What does vielä mean here, and where can it go in the sentence?

Vielä commonly means still (or sometimes yet). Here it’s “still”: the kitchen remains dark at this point. Word order is flexible, but placement changes emphasis:

Keittiö on vielä pimeä = neutral “still”
Keittiö on pimeä vielä = emphasizes still (less common in neutral writing)
Vielä on keittiö pimeä = strong emphasis / special style (often needs context)