MatrixSynapse/synapse/res/templates/style.css

html {
    height: 100%;
}

body {
    background: #f9fafb;
    max-width: 680px;
    margin: auto;
    font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, Helvetica, Arial, sans-serif, "Apple Color Emoji", "Segoe UI Emoji", "Segoe UI Symbol";
}

.mx_Header {
    border-bottom: 3px solid #ddd;
    margin-bottom: 1rem;
    padding-top: 1rem;
    padding-bottom: 1rem;
    text-align: center;
}

@media screen and (max-width: 1120px) {
    body {
        font-size: 20px;
    }

    h1 { font-size: 1rem; }
    h2 { font-size: .9rem; }
    h3 { font-size: .85rem; }
    h4 { font-size: .8rem; }
}

.error {
    color: red;
}