Добавьте данный код в Custom.css

#elLogo:hover {
    opacity: .4;
    -moz-transition-property: opacity;
    -moz-transition-duration: 1s;
    -moz-transition-delay: 0s;
    -webkit-transition-property: opacity;
    -webkit-transition-duration: 1s;
    -webkit-transition-delay: 0s;
    -o-transition-property: opacity;
    -o-transition-duration: 1s;
    -o-transition-delay: 0s;
    transition-property: opacity;
    transition-duration: 1s;
    transition-delay: 0s;
}