Basic centred webpage css

main.css

body {
    /* content width is 2x 350px */
    margin: auto calc(50% - 350px);
    color: rgba(0, 0, 0, .8);
    font-size: 16px;
    font-family: system-ui, 'Roboto', sans-serif;
    line-height: 1.6;
}

@media (max-width: 767px) {
    body {
        margin: auto 2rem;
    }
}