Hello,
You can use DOCXPath to insert a highlight tag to all w:rPr tags surrounded by the block placeholders.
To accomplish this you need to use a customQuery with the insertWordFragment method.
DOCXCustomizer, available only in Premium licenses, is the easiest way to accomplish what you need, as it allows to change DOCX styles on-the-fly.
As your username doesn't have any license tied, please send to contact[at]phpdocx.com the username or email of the user that has purchased the license.
Regards.