Override forum topics list
template_preprocess_forum_topic_list() initializes the variable with the value returned from _theme_table_cell().

global $forum_topic_list_header;

// Create the tablesorting header.
$ts = tablesort_init($forum_topic_list_header);
$header = '';
foreach ($forum_topic_list_header as $cell) {
  $cell = tablesort_header($cell, $forum_topic_list_header, $ts);
  $header .= _theme_table_cell($cell, TRUE);
}
$variables['header'] = $header;


&nbsp;

function _theme_table_cell($cell, $header = FALSE) {
  $attributes = '';

  if (is_array($cell)) {
    $data = isset($cell['data']) ? $cell['data'] : '';
    $header |= isset($cell['header']);
    unset($cell['data']);
    unset($cell['header']);
    $attributes = drupal_attributes($cell);
  }
  else {
    $data = $cell;
  }

  if ($header) {
    $output = "&lt;th$attributes&gt;$data&lt;/th&gt;";
  }
  else {
    $output = "&lt;td$attributes&gt;$data&lt;/td&gt;";
  }

  return $output;
}


The variable $variables['header'] in template_preprocess_forum_topic_list() is the same variable that in forum-topic-list.tpl.php is accessed as $header.