An area rug? Pillows? Artwork?

Sofa cover? You can get a custom one made if it is in your budget and you like the look.

Otherwise a lighter rug can really effectively break up the darkness.

You can also add some light throw pillows.

Otherwise I dont see any pictures so hard to give specific advice.

PICTURES

I like the rug, pillows, and art ideas. I'm going to add throws. I love throws! I have them in lots of different colors and thicknesses - some thin, some plush. I always keep my eyes open for throws! I get the less expensive ones from a drug store, and the more expensive ones from a department store. You can get them on Amazon also, of course. You can put them over the arm rest of the sofa (like a lot of people do). Another thing (for your particular situation) is to fold the throw in half on the diagonal and put it on the back of the sofa. That way, you can cover up some of the sofa back, and thus the darkness of it.