Hi, thanks so much, I am glad you enjoy the adventure.
If you are referring to the page border pattern: I used Affinity (Designer and Publisher) to lay out the adventure. I drew the border pattern in Designer and replicated the element down until it filled an A5 page, exported it as SVG and inserted it into a master page called "Decorations", which I then applied to most pages of the adventure. A more professional way would be to create a "Textured Intensity Brush" with the pattern - look it up on youtube, it's fairly simple to do.
hth, good luck with your own designs!