Hello,
1) Provide us with WP Dashboard access to your site to check the mentioned section and find what classes are used for these headings to improve the code.
2) Use the below code to change the color of that button only
.template-container .place-order .button {
background-color: green !important;
border-color: green !important;
}
.template-container .place-order .button:hover {
background: blue !important;
border-color: blue !important;
}
Regards