Positioning $logo

I am trying in Drupal6 to modify the .standart theme Bluemarine. I copied the folder to sites/all/themes as Newmarine. I want in the header line to separate the logo and the sitename and centralise the logo above the left sidebar.

The page.tpl.php defines three tables for header, content, and footer. The header table consists of one <tr> withtwo <td>'s, the first holding $logo and $site_name, the second holding $secondary_links and $primary_links. I defined one more <td> and moved $site_name into that <td> so that the first <td> only holds $logo. I further modified the tables so that header and content is in the same table. The purpose is to get the <td> with $logo aligned with the <td> that includes the left sidebar. And that works fine. But I cannot centralise the $logo in its <td>.

Here are the relevant lines of code as modified:

<table border="0" cellpadding="0" cellspacing="0" id="header">
  <tr>
    <td id="logo">
    <?php if ($logo) { ?><a href="<?php print $front_page ?>" title="<?php print t('Home') ?>"><img src="<?php print $logo ?>" alt="<?php print t('Home') ?>" /></a><?php } ?>
    </td>
    <td id="logo2"> 
      <?php if ($site_name) { ?><h1 class='site-name'><a href="<?php print $front_page ?>" title="<?php print t('Home') ?>"><?php print $site_name ?></a></h1><?php } ?>
      <?php if ($site_slogan) { ?><div class='site-slogan'><?php print $site_slogan ?></div><?php } ?>
    </td>
    <td id="menu">
      <?php if (isset($secondary_links)) { ?><?php print theme('links', $secondary_links, array('class' => 'links', 'id' => 'subnavlist')) ?><?php } ?>
      <?php if (isset($primary_links)) { ?><?php print theme('links', $primary_links, array('class' => 'links', 'id' => 'navlist')) ?><?php } ?>
      <?php print $search_box ?>
    </td>
  </tr>
//</table>

//<table border="0" cellpadding="0" cellspacing="0" id="content">
  <tr>
    <?php if ($left) { ?><td id="sidebar-left">
      <?php print $left ?>
    </td><?php } ?>

o.s.v.

I have tried in style.css for #logo to insert text-align:center, and I have tried to style the <td>, the <a>, and the <img> as text-align:center, and I have tried with display:block and a number of other tricks, but the displayed position of the $logo does not change. It stays aligned left.

How should I modify the code to get $logo aligned central in its <td>?