dash-hello-world.css

html {
    font-family: "Open Sans", "HelveticaNeue", "Helvetica Neue", Helvetica, Arial, sans-serif;
    font-weight: 200;
    height: 100%;
    position: relative;
    box-sizing: border-box;
    margin: 0;
    padding: 0;
}

/* plotly.js's modebar's z-index is outta the roof by default - lower it down */
.js-plotly-plot .plotly .modebar {
    z-index: 0 !important;
}

.container {
    padding: 10px;
    margin: 20px;
}

body,
div {
    box-sizing: border-box;
    margin: 0;
    padding: 0;
}

._dash-loading {
    position: absolute;
    overflow: hidden;
    box-sizing: border-box;
    display: flex;
    align-items: center;
    justify-content: center;
    flex-direction: column;
    color: #506784;
    font-size: 14px;
    height: 100%;
    width: 100%;
}

._dash-loading::before {
    content: '';
    box-sizing: border-box;
    overflow: hidden;
    border-radius: 8px;
    height: 50px;
    width: 50px;
    transform: translateX(-4px);
    background: url('https://plot.ly/static/webapp/images/plotly-logo-loop.gif') no-repeat center center;
    background-size: cover;
    margin-bottom: 10px;
}