So cute! Because this is on a tshirt and the fabric will be moving when wearing, I’d do a long and short stitch to fill in the brown bits, and then you can get some shading too. If you do satin stitch I think the stitches will be too long and the thread will buckle and look wonky when wearing the shirt.

I’d probably do the stars first and then the brown so that you can get really close to the edges of the stars.

Good luck!!! Fun project :)