I love the idea of variegated thread. Such a pretty quilt!

On this I would use black....variegated to me would take away from the colorful triangles

Wonderful quilt.

I am sure however you quilt and the color of thread will only make it more beautiful.

I have done a couple of quilts with similar colors, black background. I used black thread and for yours I would stitch in the ditch around the colored triangles. I did a kind of spiral but it was squared rather than a circle, if that makes sense.
Honestly, the quilting didn't show up a lot, but still I liked it. I didn't really want the quilting to be the "star of the show".

Your quilt is really nice. I love the brights on the black.
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Beautiful quilt! She will love it!

This is really awesome. I like artsyone's suggestion.

I use " gunmetal gray" instead of black on my Gammil, which was recommended by my dealer years ago. Always looks nice.

I had my "black" setting triangles quilted in bright colors of variegated thread. The judge loved it and the audience gave it first place.

I would quilt around the triangles and straight line the bigger black part. I like your quilt a lot.