Feel free to use any fabric you like in the center. With the kit fabrics, the overall feel of this quilt is "light". There is enough contrast in the center of the kit quilt for me. But if you prefer a darker center, that's perfectly OK too.
As we see more of these quilts coming together, you can get a good feel for what appeals to you. I always recommend quilters think about why a particular quilt appeals to them, for example, at a show or online. Is it the color, the size of print, the busy-ness or calmness of the piece, what is it about the quilt that speaks to you? I love quilts with lots of fabrics and complex piecing but sometimes a "modern" quiet piece stops me in my tracks. Learning what appeals to you the most is part of the process of quiltmaking.
Enjoy the journey.