Yeah it appears to be coming from the plugins css code –
.mm-checkoutSection .mm-purchaseSection {
clear: both;
background: #fff;
overflow: auto;
border-radius: 5px;
padding: 10px 0px 10px 0px;
}
So you would need to add some css (in your style.css file) to override it –
.mm-checkoutSection .mm-purchaseSection {
background: none !important;
}