Got it. I’m looking at the source code to try and figure it out for you but no luck yet. Even though it looks I bit familiar to me it might be a custom built theme that he may have made himself. You can also try contacting him and ask which theme he’s using. Most blog owners usually are pretty responsive to that kind question.
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He may have coded it to his needs and he may have made necessary Code changes to make it current. Or it simply still works out of the box even though it wasn’t updated. You can also Google “Casper WordPress Theme” to see if there’s another theme out there with the same name.
I didn’t dig into the theme but a good majority of themes have ways of publishing code examples and snippets. Or he added that functionality or he added a plugin for it. As I said before, reach out to him. He may surprise you with his willingness to answer your questions.