body, html {
    height: 100%;
    margin: 0;
}
body {
    display: flex;
    flex-direction: column;
    min-height: 100vh;
}
main {
    flex: 1 0 auto; /* grow and shrink as needed */
    overflow: auto; /* enable scrolling inside content if needed */
    padding: 1rem;
}
header, footer {
    flex-shrink: 0; /* don’t shrink header/footer */
}