It should be inserting some code into the template file to recognize it in the .md content text. The problem is it’ll be part of the entire content text. That means, the lightbox image can be anywhere within the text of the blog content.
It looks like if you use the build tool on the website, you can generate a single js file that you can simply add to the static directory in your project. You should then be able to do the same with the css file that it links you to.