Learn Finnish: Huone valaistaan niin, että kokonaisuus on rauhallinen. - The room is illuminated so that the whole is peaceful.

In Finnish, the known information (the Theme) usually comes first. Placing huone first shows we are talking about a specific, already known room ("The room is illuminated"). If the order were Valaistaan huone, it would mean "A room is illuminated," introducing a completely new room to the context as new information (the Rheme).

Since valaistaan is in the passive, is huone the subject of the sentence?

Grammatically, no. Finnish passive sentences do not have a formal subject; huone is actually the object in the nominative case. However, because of the Theme-Rheme principle, it gets placed at the beginning of the sentence just like an English subject would, simply to show it is the established topic we are currently discussing.

Could I use jotta instead of niin, että here?

You could, but it changes the nuance slightly. The phrase niin, että focuses on the manner and the result ("illuminated in such a way that the result is peaceful"). Using jotta would focus strictly on the underlying intention ("illuminated in order to make it peaceful"). In aesthetic contexts like this, niin, että is very natural for describing a resulting effect.

How exactly is the word kokonaisuus being used here?