Hi,
You want to edit the CSS of HikaShop via the CSS section of the Display tab of the HikaShop configuration.
There, you have two pieces of code for these button styling:
a.hikashop_cart_button, a.hikashop_compare_button, .hikashop_cart_input_button{
text-decoration:none;
color: #ffffff !important;
cursor: pointer;
display: inline-block;
line-height: 18px;
margin: 5px;
padding: 3px 8px;
text-align: center;
box-shadow:none !important;
text-shadow:none !important;
border:1px solid;
border-color: #EEEEEE #DDDDDD #DDDDDD #EEEEEE;
border-radius:3px;
background-color: #eee;
background: -webkit-gradient(linear, left top, left bottom, from(#ffffff), to(#eee));
background: -webkit-linear-gradient(top, #654f47, #654f47);
background: -moz-linear-gradient(top, #ffffff, #eee);
background: -o-linear-gradient(top, #ffffff, #eee);
background: linear-gradient(top, #ffffff, #eee);
}
This one is for the button when you don't hover on it.
And this one is for when you hover on it:
a.hikashop_cart_button:hover, a.hikashop_compare_button:hover, .hikashop_cart_input_button:hover{
box-shadow: 0 1px 0 rgba(255, 255, 255, 0.2) inset, 0 1px 2px rgba(0, 0, 0, 0.05);
display: inline-block;
outline: none;
border:1px solid;
border-color: #EEEEEE #DDDDDD #DDDDDD #EEEEEE;
text-decoration:none;
background-color: #f8f8f8;
background: -webkit-gradient(linear, left top, left bottom, from(#ffffff), to(#f8f8f8));
background: -webkit-linear-gradient(top, #ffffff, #f8f8f8);
background: -moz-linear-gradient(top, #ffffff, #f8f8f8);
background: -o-linear-gradient(top, #ffffff, #f8f8f8);
background: linear-gradient(top, #ffffff, #f8f8f8);
}