.open-header {
    background-image: url(/images/event/201711/tsb_112017_content_tw.png);
}

.signin {
    background-image: url(/images/event/201711/tsb_112017_btnlink_tw.png);
}

.signin:hover {
    background-image: url(/images/event/201711/tsb_112017_btnhover_tw.png);
}

@media screen and (max-width: 640px) {
    .open-header {
        background-image: url(/images/event/201711/tsb_112017_mweb_content_tw.png);
    }
    .signin {
        background-image: url(/images/event/201711/tsb_112017_mweb_btnlink_tw_04.png);
    }
    
    .signin:hover {
        background-image: url(/images/event/201711/tsb_112017_mweb_btnhover_tw_04.png);
    }
}