"Because the room is dark, let's turn on the light."
部屋が暗いですから電気をつけましょう I'm dead sure this is correct, but Duo doesn't think so.
ですから and だから should both be accepted, and are in fact a better way to say this as they remove ambiguity.
Usually I see「だ」instead of「です」in the middle of a sentence. Polite forms are used at the very end.
Ah, the old は/が distinction. In short, both would be acceptable here but が has the nuance of "the room but maybe not other rooms." The から particle, to answer your other question, in this case means "because," and cannot be changed to がら (you probably just mistyped it though). Lastly, saying くらいへや would mean "dark room" instead of "The room is dark," so the phrase would be changed to "because it is a dark room, let's turn on the lights." Which sounds a little strange, despite being correct.
Just now got this far, can anyone tell me why "because" is mentioned in the middle of the sentence?
you may have already seen から and まで for locations.
[x]から[y]まで from location x to location y
Similar with reasoning just without using "made" we now have
[x]から、電気をつけましょう。from reason x we deduct we should turn on the light