seahub/media/css/spreadsheet_convert.css

#spreadsheet-container iframe {
    width: 950px;
    background:#fff;
    border: 1px solid #ccc;
    box-shadow: 0 0 6px #ccc;
    margin: auto;
    min-height: 500px;
}
@media (max-width: 991.98px){
    #spreadsheet-container iframe {
        width: 100%;
    }
}