.ui-button .ui-icon.icon-outlook {
    background-image: url(icons/outlook.png);
    width: 16px;
    height: 16px;
    background-size: contain;
}

.ui-button.ui-state-hover .ui-icon.icon-outlook {
    background-image: url(icons/outlook.png);
    width: 16px;
    height: 16px;
    background-size: contain;
}