How to style current page number (wp_link_pages)?

When a post is split on more pages TwentyTen theme use the native function wp_link_pages to display a navigation page bar at the end of post.

I am trying to style those elements for my theme, but unfortunately it seems that the current page number cannot be styled.

I imagine that I should override the wp_link_pages function but I am still learning the basic of WP programming.

Can you help me identifing the steps to follow to solve this problem?

Unfortunately, there is no way to do this just with native functions: WP is … request agnostic and produces always links to the current page (nav manus, list pages …).

Also, you cannot use a filter, because wp_link_pages() has no appropriate filter.

In my themes, I use an own function, based on this code. It is probably too long to post it here, so I put it as a plugin on GitHub: Logical Page Links.
You may use the plugin as is or copy the code into your theme.

The resulting markup will look like this:

<p class="pager"> 
  <b title="You are here.">1</b>  
  <a class=number href="http://example.com/page/2/">2</a> 
</p>

The <b> marks the current page, you can style it via:

.pager b
{
    color:      #fff;
    background: #111;
}

More features are listed in the readme of the plugin.

Update

I misunderstood the question. I thought you needed such a function for archives. Sorry.

Here is a rewritten version of wp_link_pages() as a plugin. I guess you’ll put it into your theme.

<?php # -*- coding: utf-8 -*-
/*
Plugin Name: Numbered In-Page Links
Description: Replacement for wp_link_pages with numbers. Use do_action( 'numbered_in_page_links' );
Version:     1.0
Required:    3.1
Author:      Thomas Scholz
Author URI:  http://toscho.de
License:     GPL v2
*/
! defined( 'ABSPATH' ) and exit;

add_action( 'numbered_in_page_links', 'numbered_in_page_links', 10, 1 );

/**
 * Modification of wp_link_pages() with an extra element to highlight the current page.
 *
 * @param  array $args
 * @return void
 */
function numbered_in_page_links( $args = array () )
{
    $defaults = array(
        'before'      => '<p>' . __('Pages:')
    ,   'after'       => '</p>'
    ,   'link_before' => ''
    ,   'link_after'  => ''
    ,   'pagelink'    => '%'
    ,   'echo'        => 1
        // element for the current page
    ,   'highlight'   => 'b'
    );

    $r = wp_parse_args( $args, $defaults );
    $r = apply_filters( 'wp_link_pages_args', $r );
    extract( $r, EXTR_SKIP );

    global $page, $numpages, $multipage, $more, $pagenow;

    if ( ! $multipage )
    {
        return;
    }

    $output = $before;

    for ( $i = 1; $i < ( $numpages + 1 ); $i++ )
    {
        $j       = str_replace( '%', $i, $pagelink );
        $output .= ' ';

        if ( $i != $page || ( ! $more && 1 == $page ) )
        {
            $output .= _wp_link_page( $i ) . "{$link_before}{$j}{$link_after}</a>";
        }
        else
        {   // highlight the current page
            // not sure if we need $link_before and $link_after
            $output .= "<$highlight>{$link_before}{$j}{$link_after}</$highlight>";
        }
    }

    print $output . $after;
}