You can add it to the a.button.small
section of your css around line 529 like this –
a.button.small {
border-radius: 4px;
background-color: #303236;
color: #fff;
font-size: 13px;
font-weight: bold;
line-height: 13px;
padding: 8px 20px;
}
and then you can adjust the radius if you want. You can also add this to the medium and large buttons in the css as well (if you want).