Change appearance based on category but post is in two main categories

One easy way is to use has_category function.

E.g. in your header you can use

if ( has_category('pirtek') ) {
  $header="/path/to/pirtek/header"
} else {
  $header="/path/to/standard/header"
}

If the post has the ‘kirtek’ category, then the condition inside if is true, no matter what other categories the post belongs to.

This kind of if statement can be used everywhere you need, however, the snippet above works well in singular templates and inside the loop. If you want to use that conditional outside the loop, you need to pass the post object as second argument of has_category.

$postid = 10;

if ( has_category( get_post($postid) ) ) {
  // do something 
}