A large table lamp or wall sconce in each corner of each room, at chest height. Other ways to add light are (if this goes with their style), to add reflective surfaces such as glass table tops and mirror lamp bases, picture frames, and/or accessories. Also, make sure the majority of the colors are light. Also, have eggshell paint finish on the walls.

Check to make sure that the most reflective materials and colors are used in the home.

• Are the ceilings painted white?
• Are the furnishings selected reflective and light? Nothing swallows light easier than a dark wall to wall carpet.
• What color are the wall painted?
• Are the window coverings minimal and allowing light to enter?
• Have you considered picture lights over art?

An electrician can add wall sconces.

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If your parents prefer an inexpensive option, buy four $20 can uplights, put one in each corner. https://www.amazon.com/Gloss-White-Adjustable-Accent-Uplight/dp/B0000DI4D2  Uplights that are built into the floor would be a more expensive option.

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Cove lighting with LED strip lights behind molding on the top of the walls may work.

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If the lighting is a big problem rather than a small inconvenience, consulting a professional lighting designer would be a good idea.

Table lamps, floor lamps, wall lights. Some ceiling lights have long cords, so the light can be hung from the ceiling and plugged into a wall outlet. Also as noted above, paint is your friend.

Surface mount raceways can make low height LED flushmount ceiling fixtures possible. You just haven't talked with the right electrician or designer that knows how to make that work and look good.