Hello,
Go to Theme Options > Custom CSS > find the code below and remove it
.single_add_to_cart_button.button {
font-family: Syncopate;
height: 2.3rem;
padding: 0;
font-weight: 400;
font-size: 15px;
background: none;
border-bottom: 1px solid #545454;
border-left: 0;
border-right: 0;
border-top: 0;
color: #545454;
letter-spacing: 0;
margin-right: 0.4rem;
min-width: auto;
}
Regards