Set a specific style with Helix depending on a server

Hellow
We would like to use a specific template style for Helix when the server returns a specific header.
The code would be :

$template_style = ???
function getheaderms($name)
    {

    $namesrv='HTTP_'.str_replace(' ', '_',strtoupper(str_replace('-', ' ',$name)));
   return $_SERVER[$namesrv];

    }
if ( isset( $_SERVER["HTTP_CERCLE_VERSION"] ))
{$value=getheaderms('Cercle-Version');}

//$value=getheaderms('Cercle-Version');

if( isset($value)) {return $template_style; }

The idea would be to insert this code as early as possible.

Can you tell me how to get in PHP a specific helix template style and where to include this code ?
Would it be best to include it in all helix template styles or to do an override somewhere and where ?

thanks for your help
Best regards
Jean