Tutorial 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. Gotta add a function to functions.php:
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"';
}