"Because the room is dark, let's turn on the light."
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.