I would like to know where I can find and edit the code that highlights the keywords in the results.
I looked in PHP files and did not find anything. So maybe this is in JavaScript.
It is done via javascript, and by default it is minified and combined so it is not easy to modify.
If you wish to change it, first make sure to change this option.
Then edit the file: wp-content/plugins/ajax-search-pro/js/nomin/jquery.highlight.js