style.css

.mjml-block {
    background-color: #f5f9f9;
    border: 1px solid;
    border-color: #c4ced2 #dce7ed #dce7ed;
    padding: 10px;
}

.mjml-block.error {
    color: #ee1a1a;
}

.mjml-tr > td:first-child {
    width: 50%;
}