html {
    /* Increase from the default 16px to 18px or 20px */
    font-size: 18px; 
}

body {
    /* "Libertinus Serif" is the specific name from the CDN above */
    font-family: "Libertinus Serif", serif !important;
}

/* Optional: Ensure Bootstrap headings use it too */
h1, h2, h3, h4, h5, h6, .h1, .h2, .h3, .h4, .h5, .h6 {
    font-family: "Libertinus Serif", serif !important;
}