Hello,
The fixed header structure is independent of main header type. Fixed header code can be modified in the wp-content/themes/woopress/header.php file (please do changes via child theme https://codex.wordpress.org/Child_Themes). Please note that changes in php files are additional customization in source code and it is out of our support scope. If you need help with additional customization contact WP-Kraken team.
Regards