@media screen and (max-width: 1367px) {
    /*果使用者之視窗寬度 <= 1367，將會再載入這裡的 CSS。*/
    #back-top {
        right: 20px;
        position: fixed;
        bottom: 30px;
        display: none;
    }
}

@media screen and (min-width: 1368px) {
    /*果使用者之視窗寬度 > 1368，將會再載入這裡的 CSS。*/
    #back-top {
        right: 10%;
        position: fixed;
        bottom: 30px;
        display: none;
    }
}

#back-top a {
    width: 54px;
    display: block;
    text-align: center;
    font: 11px/100% Arial, Helvetica, sans-serif;
    text-transform: uppercase;
    text-decoration: none;
    color: #bbb;
    -webkit-transition: 1s;
    -moz-transition: 1s;
    transition: 1s;
}

#back-top a:hover {
    color: #000;
}

#back-top span {
    width: 54px;
    height: 54px;
    display: block;
    margin-bottom: 7px;
    background: #ddd url(../images/up-arrow.png) no-repeat center center;
    -webkit-border-radius: 15px;
    -moz-border-radius: 15px;
    border-radius: 15px;
    -webkit-transition: 1s;
    -moz-transition: 1s;
    transition: 1s;
}

#back-top a:hover span {
    background-color: #777;
}