You have a great quilt ... and I don't feel any of your options are doing justice to what you have done and will only detract from what you already have as they aren't colours that are in the quilt.
Yes, perhaps the light blue is the background in the horse print ... can't tell from the picture here. Though even if it is, it doesn't help frame up the work as you have shown it. If it has, do a narrow (thinner than the green/gold borders) border. Then gold or green ... or do steps and use both.