wordpress: add class to prev/next posts links

functions.php

//NEXT/PREV BLOG PAGE LINKS
add_filter('next_posts_link_attributes', 'posts_link_attributes_1');
add_filter('previous_posts_link_attributes', 'posts_link_attributes_2');

function posts_link_attributes_1() {
    return 'class="next btn-gray"';
}

function posts_link_attributes_2() {
    return 'class="prev btn-gray"';
}