Loads and displays the appropriate header data for a theme.

• Sends an "Expires" header of "Mon, 26 Jul 1995 05:00:00 GMT" to prevent caching
• Loads the "{layer}_above" sub-template for each layer in $context['template_layers']
• Outputs the security warning to remind the admin to remove install.php, upgrade.php, repair_paths.php and repair_settings.php
• If the person browsing has triggered a ban, outputs a notice stating so and giving the appropriate reason
• Sets $settings['theme_url'], $settings['images_url'] and $settings['theme_dir'] to the default theme's values if $settings['use_default_images'] is set to 'defaults'