Paint the walls a lighter shade of brown or a neutral color that complements the dark brown curtains without creating a dark or dingy look. Cream, beige, buttery yellow or white are a few options. Consider wood paneling or deep red or light brown weathered-looking walls if you plan to use the living room as a formal library or man's den.
Furnish the room with pieces that include dark brown details and lighter shades to complement the curtains. Dark leather or cream-colored microfiber sofas will work, depending on how light or dark you want the living room to feel. Glass tables will visually open the space, while dark wood will play off light-colored furniture and tie in with the dark brown curtains.
Add other textiles that complement the dark brown curtains. Dark brown throw pillows, patterned rugs or a throw across the back of the sofa or over the ottoman will unify the space. Alternatively, dress up the curtains with reflective pieces of copper, crystal bead tiebacks or chrome curtain rod and finials. Pick up the metallic finish elsewhere in the room if you go this route, such as in accessories or the tables.
Hang colorful artwork or a gallery of photographs or metallic wall hangings. Use dark brown frames for artwork, pictures or mirrors to play off the curtains.
Accessorize the room with a bright color mixed in with neutrals or use dark brown as an accent color. For example, pair the dark brown with cream colored furniture and robin's egg blue pillows, lamps, candles and vases. Use patterns that include all three colors on the rug, throw pillows or artwork.