How about painting them? It is a big job, but worth it in the end.
I'd brighten them up with some white or light gray paint (or whatever goes with your decor). Here are some tips/ways to paint cabinets on Hometalk: https://www.hometalk.com/search/posts?filter=painting%20cabinet
Take one of the cabinet doors to the paint department of the home improvement store and ask them to select paint/stains and other finishes and application tools that will work for your kitchen. They have the best selection, the most up-to-date materials and most important, the most informative sales staff to get you what you need for a professional job.
Just paint them a lighter color
I thought this was pretty cool - cover them in fabric! https://www.hometalk.com/27279359/renter-s-cabinet-cover-up?expand_all_questions=1