Paint the walls with light colors that brighten the room and take the focus off the darkness. Light slate blue or mint green contrast well with brown and bring out the grain in the wood furnishings. Wallpaper one wall with a geometric pattern in shades of yellow, green or blue to add a unique aspect to the décor.
Pair the brown leather couch with chairs upholstered in textured fabrics in shades of blue, green or yellow. If you like a lot of leather in the room, choose white or cream colored leather overstuffed chairs and ottomans. If the sofa is bulky and heavy, pair it with furniture with lighter lines to balance the room. Bring attention to the existing wood furniture with end or coffee tables made of glass, brass or silver, which also complement the rich tones of the leather sofa.
Toss a few brightly colored pillows on the sofa in assorted shapes and unique designs. Stack some animal pattern floor pillows in the corner to add a whimsical touch to the earth tones. You can soften the look and lines of a brown leather sofa with a lightly woven silk throw draped over the back or casually arrange small throws in light fabrics and colors on the arms of the sofa.
Lighten up the room with airy window treatments such as sheer panels or gauze draperies. Use colorful strands of wide satin ribbon or metallic or jeweled tiebacks. If privacy is not an issue, forego curtains and decorate the windows with colorful valances made from materials with tiny strands of metallic thread running through them. Install red or lemon colored Roman shades on windows to lighten the atmosphere and provide protection from light and neighbors.
Hang paintings and pictures with colorful themes to lighten up the space. Cover some of the wood furniture with sheer or lace doilies or cloths to make them more interesting and display an array of curios that mix modern knickknacks with antique pieces. To tie the room together, display a small leather-covered table or stool in a corner of the room a good distance from the couch.