If you are doing it on a longarm you can do a meandering, with outlining or shadowing around the main design inside the t-shirts. Thread - either invisible or top thread to match the shirts, and maroon to match the back, but watch your tension so that either color doesn't show on the opposite side.
Your main goal in a t-shirt quilt is to show off the t-shirts and not the quilting, while holding the quilt together and enough stitches so it doesn't bunch.
Have fun. It looks good so far.
Phyllis
QuiltingGrannie
Quilter's Pantry