For your son's room, a mural or wallpaper would be nice and add a side table and lamp to warm up the room. Given you are renting, it may have to be removed before you leave. If that's a problem, large posters can do the trick as well. This mural is from a Canadian company NumérArt. It is pre-pasted wallpaper available in 2 sizes: (12' wide x 8.5' high) or (16' wide x 10.5' high).

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In your room, layering the bed will make a big difference. I would purchase a white coverlette and fold down your comforter and layer the pillows and art would help. Not sure of your budget, but looking at beige and black master bedrooms may also provide you with some ideas on accessorizing (ie: bench at bottom of bed, an additional nightstand, etc.). Given it is a rental, you may not want to add drapery to both rooms, but that would be nice also.

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Adding side panels to both windows will add a visual height to the space. Install your hardware as close to the ceiling as possible. Buy longer premade panels and hem them to your height. Use dark hardware and rings for your window treatments in both rooms, those will tie nicely with your dark furniture.

Remember when you are working with tone on tone colors you need to mix different textures. Fur throw, bolster pillow, one more nightstand with a lamp and of course artwork with dark frame will do wonders to this space.

Like the lamp you picked.