imo, a flower would be too fussy for that quilt. it's a very masculine looking one and should be quilted with a simple design.
either 1/4" inside the larger square or a circle inside the larger square. i would sid around the rectangle that encloses the small triangles.
what size are the large and small triangles and the quilt? and are you hand- or machine- quilting it?
i think medium brown thread. it will show, but not take over.