OpenSlides/openslides/static/styles/agenda.css

tr.activeline a.activate_link div {
    background-image: url(/static/images/icons/task-accepted.png);
    background-repeat: no-repeat;
    background-position: center;
    width: 20px;
    height: 20px;
}

tr.inactiveline a.activate_link div {
    background-image: url(/static/images/icons/task-accepted-grey.png);
    background-repeat: no-repeat;
    background-position: center;
    width: 20px;
    height: 20px;
}


a.close_link.closed div {
    background-image: url(/static/images/icons/user-offline.png);
    background-repeat: no-repeat;
    background-position: center;
    width: 20px;
    height: 20px;
    display: inline-block;
}

a.close_link.open div {
    background-image: url(/static/images/icons/user-online.png);
    background-repeat: no-repeat;
    background-position: center;
    width: 20px;
    height: 20px;
    display: inline-block;
}