Add class to links generated by next_posts_link and previous_posts_link

These two functions create anchor links, and you can customize a lot of it, but it's impossible to add a class through just using their parameters alone. Use the themes functions.php file to give these links a classname:

add_filter('next_posts_link_attributes', 'posts_link_attributes');
add_filter('previous_posts_link_attributes', 'posts_link_attributes');

function posts_link_attributes() {
    return 'class="styled-button"';
}