Agree with Margo. Match the thread to the background. It looks (as best I could see on the web site) like they used the same all over quilting in the panels. Looks like the geese were outlined, not sure about the block panels.

Julia, you can use a very fine thread (Superior's So Fine brand or MasterPiece cotton) that matches the background and just stitch right over the embroidery. I have no idea how much detail you want to add, but here are a couple of samples of a different Crabapple Hill design that have fabulous quilting:

Advice needed please! I'm just finishing a design by Crabapple Hill called "Over the river and through the woods to grandmother's house we go". It has three panels of embroidery, two pictures and one lettering in a backstitch. It's surrounded by flying geese and small blocks. I'd like to machine quilt but not sure how, do I use an invisible thread? I'd really appreciate any help, Julia