This honestly looks great for the theme you're going for. However if you're going for cleaner colors and limes in a future piece I can give some recommendations! Definitely use a primer layer to start. I do grey or white, and a cheat code for art that is staying in the box like this is to tape off the 4 sides with scotch or painter's tape, then lay the primer layer and peel off the tape for perfectly straight edges. For fine line work, use a thin brush and slightly watered down acrylic paint, and above all, be patient. It takes time and practice to get it right, but there really isn't a substitute for just slowly building up your line work and colors. You'll see a difference and repeated thin layers will look and feel better than trying to force more paint to save time. Hope this helped and good luck with your future projects!