By default, this will be controlled in Theme Options> Basic Styling, Primary Color. If you want to override the button and style it separate from your Basic Styling, add this CSS to Theme Options> Custom CSS:
div#inner-wrap #payment #place_order { color: #000; background: #8b909a; -webkit-transition: background-color .4s ease-in-out; -o-transition: background-color .4s ease-in-out; transition: background-color .4s ease-in-out; } div#inner-wrap #payment #place_order:hover { color: #8b909a; background: #000; -webkit-transition: background-color .4s ease-in-out; -o-transition: background-color .4s ease-in-out; transition: background-color .4s ease-in-out; }
Customize the CSS as desired.