Hi,
Add below code to Theme Options -> Custom CSS -> Global Custom CSS
.btn-black, .btn.black, .btn-checkout, .btn-view-wishlist, .form-submit input[type="submit"] {
background-color: rgb(46, 50, 130);
border-color: #2f3382;
color: #fff;
border-radius: 30px
}
Regards,
Hung PD