You could always stain it - careful though! Mind sharing a picture?

Not sure why the pic didn't go through the first time...

I could see two approaches here:

Either paint it the same colour as the wall or one shade darker, or paint it very dark grey or green-black. In both scenarios I would then add a large painting in a frame that fills nearly the entire space, leaving a 4-6" border of the painted wood around it.

Thanks for the input. I agree, I think darker is the way to go. Paint the shelves the same?

Yes to painting the shelves the same, the only caveat being if it original wood I am always anti-paint :-D

Agreed, definitely not original wood so I'm fine painting it. My molding is the same dark wood as the built-in. Still debating if I should paint them white...

you could also do a wallpaper on the back.

Paint them in white

I would redo the opening, and take it back to the original by removing the filler. It appears it would look great with the way you have your furniture arranged, and it would add a lot of charm to the space.

Hey Sammie, I agree but...my house is actually a single family converted to a multifamily. Right now that wall is separating the living room from my bedroom and I'm renting out the upstairs. If I ever convert it back to a single family though, I'll definitely open it back up