Adding an Unlinked Space in a Custom Function

In my opinion CSS isn’t the right way, it should be a “real” space.
Also there is no class to target the differents links in the string.

Try this code:

    function wp_link_pages_args_prevnext_add($args)
{
    global $page, $numpages, $more, $pagenow;

    if($page-1) # there is a previous page
        $args['before'] .= ' '. _wp_link_page($page-1) 
            . $args['link_before'] . $args['previouspagelink']  . '</a>' . ' '
        ;

    if ($page<$numpages) # there is a next page
        $args['after'] = ' '. _wp_link_page($page+1)
           . $args['link_before'] . $args['nextpagelink']  . '</a>'
            . $args['after'] 
        ;

    return $args;
}