OpenSlides/openslides/participant/static/styles/participant.css

a.status_link.active span {
    background-image: url(../images/icons/user-online.png);
    background-repeat: no-repeat;
    background-position: center;
    width: 16px;
    height: 16px;
    display: inline-block;
}
a.status_link.inactive span {
    background-image: url(../images/icons/user-offline.png);
    background-repeat: no-repeat;
    background-position: center;
    width: 16px;
    height: 16px;
    display: inline-block;
}