Theme cannot be installed in /sites/default/themes

The include call in page.tpl.php seems to always look for files within /sites/all/themes, even if the the theme is installed elsewhere (e.g. /sites/default/themes). This results in an execution error and failure to render the page content and sidebars.

Changing the call to use the $theme_path variable to get the full path to the installed theme corrects the issue:

  include $theme_path . 'page-' . $a .'.tpl.php';