@media (min-width:769px) {
.main-navigation .main-nav ul li.online-button a {
background-color: #ffffff;
border: 2px solid #000000;
color: #000000;
line-height: 35px; /*this number will likely need to be adjusted*/
}
}
Adjust you colors etc to what your need
If you need further help – share a link to your site and explain the style you need.