prometheus/web/ui/static/css/config.css

.btn {
    border-radius: 0;
}

#config_yaml {
    display: block;
    padding: 9.5px;
    font-size: 13px;
    color:#333;
    word-break: break-all;
    background-color: #f5f5f5;
    border: 1px solid #ccc;
    border-radius: 4px;
}