Please kindly describe in more details what do you want to do with that? The X Theme is not coded with the traditional way as it has four different stacks, it has four different header functionality.
If you want to add something into the
<head> tag of the code you can use the
wp_head action in your Child Theme. Please click here to find out more about the Child Theme, and here on how to add the
wp_head action into the
functions.php file of the Child Theme.
I'd suggest that you bookmark this page which is the list of all documentation of the theme.