Custom Javascript function

Hi, I have a site that loads up a game with one or many text pages, each page is unique. So whatever happens on page 1 stays on page 1 and so on. We use a library to create highlights, but for some reason it is not working properly. You can maybe take a look and try and solve the existing solution i If not I need help creating a small javascript function that does the following: track user selection, and set a yellow background to that selection. That selection will be saved to a API that is ready, and when you refresh page you must be able to load the selection to see the highlighted text. This must happen for each of those pages. If we can add more color it would be better.