I like backings with interest and life to them, so to me that's a perfect one!
As for the thread ... audition them, just as you do with fabrics.
An easy way to do it is to pull off a few yards of the thread, and let it fall onto the quilt, much like the quilting lines would be. Stand back and take a look. Black or Grey as you are already considering ... I would even try the same colour as your backing. And for more interest ... look at variegated threads ... grey/black ... and you might even find one that mixes in some of that turquoise of the backing.
It's a beautiful quilt!