functions.php

<?php

/**
 * Previous/Next links as button 
 * @author Bill Erickson
 * @link http://www.billerickson.net/code/previous-next-links-as-button
 *
 * @param string $attr, link attributes
 * @return string
 */
function be_pagination_attr( $attr ) {
	$attr .= ' class="button"';
	return $attr;
}
add_filter( 'previous_posts_link_attributes', 'be_pagination_attr' );
add_filter( 'next_posts_link_attributes', 'be_pagination_attr' );