The styles of the buttons is found in app/css/scss/elements/_buttons.scss and it contains different modifiers such as:

.btn {
 font-family: $menuFont;
 font-weight: 700;
 text-transform: uppercase;
 font-size: 11px;
 line-height: 20px;
 padding: 20px 30px;
 letter-spacing: 1px;
 transition: background-color .15s ease-out;
 border-radius:0px;
 margin-bottom: 30px;
 &--round {
 border-radius: 3px;
 }
 &--line {
 border: 2px solid #e5e5e5;
 }
 &--line:hover {
 color: #000;
 }
 &--white {
 background: #fff;
 color: #555;
 }
 &--black {
 background: #000;
 color: $white;
 }
...
}