Put a large mirror on a wall. Glue molding vertically and horizontally on mirror to make it look like a window. Install decorative curtain rod above and hang a pair of stylish drapes to finish the look. Lighting will bounce off the mirror (window) and reflect back into the room giving the illusion of a real window.
Or try a lighted fake window: https://www.bobvila.com/articles/genius-the-easy-cure-for-every-dark-hallway/
You can lighten the room by adding mirrors either big ones or square ones in a set. Also, you can add light wall hangings too like decorative rugs/cloth posters made for hanging
Mirrors and lighting can solve those issues. Are there any windows in the room? How is the room situated? North, South, east or west?
daylight LED bulbs -- will drastically brighten a space