How do I decorate a room that lacks proper light?

You could add some fun lamps to brighten up the space. They add cozy vibes too. I have a whole round up of lamps here: https://www.aratariathome.com/blog/farmhouse-style-lamps-round-up

You could also paint the room a light color to brighten it up.

Hello. I would keep the colors neutral and light. Then add a mirror and a nice tall lamp. That would brighten it up.

Add lamps and mirrors.

You can always add lighting! Paint the walls light colors. Use lots of mirrors as they bounce light around the room. Curate metallic or shiny decor objects. Have an accent color in a bright color scattered around the room. Pillows, throws, artwork, etc.