editor.js/plugins/video/embed.css

.ce-redactor .embed {
    width: 100%;
    max-width: 650px;
    margin: 10px auto;
}

.embed__loader {
    background-image: url("loading.gif") !important;
    background-repeat: no-repeat;
    background-position: center center;
    opacity: 0.5;
}