It's best if you stick to editing the theme.php file in your theme. This way when you upgrade, you won't have to remember all the other hacks that you did on any core files. To answer the question in the first post, you can edit the $template_tab_display variable like below. If you don't have $template_tab_display in your theme.php, copy it from the sample theme.php and paste it into yours.
// Template used for tabbed display
$template_tab_display = array('left_text' => '<td width="100%%" align="left" valign="middle" class="tableh1_compact" style="white-space: nowrap"><b>{LEFT_TEXT}</b></td>' . "\n",
'tab_header' => '',
'tab_trailer' => '',
'active_tab' => '<td><img src="images/spacer.gif" width="1" height="1" alt="" /></td>' . "\n" . '<td align="center" valign="middle" class="tableb_compact"><b>%d</b></td>',
'inactive_tab' => '<td><img src="images/spacer.gif" width="1" height="1" alt="" /></td>' . "\n" . '<td align="center" valign="middle" class="navmenu"><a href="{LINK}"><b>%d</b></a></td>' . "\n",
'inactive_prev_tab' => '<td><img src="images/spacer.gif" width="1" height="1" alt="" /></td>' . "\n" . '<td align="center" valign="middle" class="navmenu"><a href="{LINK}"><b>{PREV}</b></a></td>' . "\n",
'inactive_next_tab' => '<td><img src="images/spacer.gif" width="1" height="1" alt="" /></td>' . "\n" . '<td align="center" valign="middle" class="navmenu"><a href="{LINK}"><b>{NEXT}</b></a></td>' . "\n",
);
and remove the {LEFT_TEXT} tag. Edit the html code to give the divider line to suit your taste.
// Template used for tabbed display
$template_tab_display = array('left_text' => '<td width="100%%" align="left" valign="middle" class="tableh1_compact" style="white-space: nowrap"></td>' . "\n",
'tab_header' => '',
'tab_trailer' => '',
'active_tab' => '<td><img src="images/spacer.gif" width="1" height="1" alt="" /></td>' . "\n" . '<td align="center" valign="middle" class="tableb_compact"><b>%d</b></td>',
'inactive_tab' => '<td><img src="images/spacer.gif" width="1" height="1" alt="" /></td>' . "\n" . '<td align="center" valign="middle" class="navmenu"><a href="{LINK}"><b>%d</b></a></td>' . "\n",
'inactive_prev_tab' => '<td><img src="images/spacer.gif" width="1" height="1" alt="" /></td>' . "\n" . '<td align="center" valign="middle" class="navmenu"><a href="{LINK}"><b>{PREV}</b></a></td>' . "\n",
'inactive_next_tab' => '<td><img src="images/spacer.gif" width="1" height="1" alt="" /></td>' . "\n" . '<td align="center" valign="middle" class="navmenu"><a href="{LINK}"><b>{NEXT}</b></a></td>' . "\n",
);