Hello,
Inside sources/template/modules/modules_header et modules_footer you will have the information
For the header I recommend in first to copy and rename the file, Like that you can change inside the administration the template you to use.
After you can modify everything.
Another way is to create a new template name and copy the css and the modules inside. In this case you must to change the template name inside you administration.
Let me know if you want more information.