Keeping baby warm in car seat

I'm a FTM and my MIL is trying to give me all this stuff and I'm really not sure if I need all of it. One of them being is a pretty heavy duty looking car seat cover. Our son is due in March and we are anticipating it not being super cold. what do you do to keep your babies warm? Do you recommend having a carseat cover? If so, any ones that you like? My plan was to dress him appropriately for the weather and maybe use a blanket above the straps to keep him warm on chilly days.