editor.js/plugins/twitter/twitter.css

.ce-redactor .twitter-tweet {
    margin: 15px auto !important;
    width: 100% !important;
}

.ce-twitter__caption {
    max-width: 520px;
    margin-bottom: 15px;
    background: #fff;
    border: 1px solid #ebeef3;
    border-radius: 3px;
    padding: .6em .8em;
    box-sizing: border-box;

}

.ce-twitter__caption:empty::before {
    content : 'Подпись';
    opacity: .5;
}