I just updated my first production site to Drupal 6. (Actually, sugree did it) Since I use custom theme, I need to update it to 6.x format.

Drupal.org provides good documentation. However, it's too long and aims for fully customized themes. My theme is based on Arc Materia and very simple. Here is my simplified version on theme updating.

1. Add new (themename).info file in directory. (more detail)
2. Change 3 region names in page.tpl.php (more detail):
   • $sidebar_left to $left
   • $sidebar_right to $right
   • $footer_message to $footer
3. Add new signature code in comment.tpl.php (more detail)
4. In very first line of page.tpl.php, change all $language to $language->language (more detail)

That's all.