A different header per category (I tried everything!)

https://jobs.wordpress.net/

The most basic solution would be:

<?php if(is_category(Specify category ID)) {?>
//Do stuff here...
<?php } else { ?>
//Do stuff here if visitor isn't viewing category X
<?php }?>

I just put that stuff in my archive.php. Unfortunately it does not work within the single.php. Are there any improvements i can make?

for a single post you need to use in_category instead of is_category

nice! works and thanks!