css (styles): various styles improvments: - work on list - new paired-slides class - box-medium is a larger than before - use --text modified when size is 1em - added several list modifiers classes - added several box modifiers

#body {
    display: flex;
    align-items: center;
    justify-content: center;
}

#link_section {
    font-weight: bold;
    text-align: center;
    font-size: 1.5rem;
}

article p:last-child {
    text-align: center;
}