Is the trim in your space white? Do you regularly use the fireplace?

I'm imagining adding some small whiteish tiles on the face of the fireplace and painting the mantel and surround white like your trim. Then, I think you would be happy with a gorgeous fireplace screen to mute the cave.

Would not change the tiles within the fireplace surround. Add an attention getting screen to your fireplace -- bright metal and/or with stained glass.

http://www.plowhearth.com/large-tree-of-life-fire-screen-with-door.htm

http://www.plowhearth.com/large-tree-of-life-fire-screen-with-door.htm

If you don't actually use your fireplace, you could create a "faux fire" -- paint the back of the interior bright red (and/or add LED lights, with or without "logs").

https://www.google.com/search?q=fake+fire+in+a+fireplace+images&tbm=isch&tbo=u&source=univ&sa=X&ved=0ahUKEwigwefEhfTZAhXHu1MKHexaDq4QsAQIVQ&biw=1366&bih=650

With you on the cost part! The mantel needs to lighten up, & that may solve the problem of painting the tile. Your art looks monochromatic, painting the mantle and repositioning the art will highlight the entire height of the fireplace. I would hang the stained glass above the mantle- off center, (window view looks pleasing) with smaller art work around & above it, some of the pieces surrounding the TV on the mantel. If painting the firebox is a go- do it, different height black iron candle holders (an odd number-5,7?) & a simple screen- think iron gate section- will highlight & disguise @ the same time. Your room looks comfy to me. good luck!