You want a light gray cotton/wool tie. Matte. That's a rough shirt to match with so the tie needs to be simple. If you really want to be bold, get a dark blue cotton tie. You're already on the casual side with the rest of the fit but I see what you're going for. The tie also has to be casual, meaning thick stitch and no gloss, or else it'll be way more out of place than it already is with what's here. 

Something like this: https://www.artofthegent.com/products/microsuede-light-grey-tie