Hey! I ended up figuring it out myself. I didn't need to add any code. I was in the shop editor, went over to theme settings (in left menu) and under corner radius set everything to 0 px, which made everything rectangular 

I am looking to edit my website, so that the Newsletter sign up space (where the name & email address go) are square rather than rounded to fit with the rest of the website. I have attached a photo to show what it looks like now. Thank you for your h...