Wanting to change the Buy now/ add to cart button

Just getting started with Marketpress and have run into some difficulties with the add to cart/buy now button.

Ideally I would want to change the button for something new entirely. If this is not possible I would like to change the colour of the button.

I was wondering where I could do this?
The site in question is http://www.uncleshelbyvintage.co.uk/store/products/ I want to change both the buttons in products page and in the product listing page.

Many thanks
Michael Barr