add custom HTML css with js doesn’t work

You cannot use the $ selector freely like this, because WordPress loads jquery in noconflict mode. The most common workaround is to wrap all functions in your script file:

jQuery(document).ready(function($) {  ... scripts ... }

If you want to put the script directly in the head of the file (not recommended) it would go like this:

<script>  
   jQuery('p.content').addClass('highlight'); 
</script>