Just to clarify the reason why (in case SS is listening), our company sells unique, one-of-a-kind products, which we REMOVE after they sell. So when categories sell out, then we have an empty category, although we don't want to remove/hide that category.
Regarding your code for the background and the script for an empty product list, both of those worked to an extent. There isn't a white background anymore, which is, at a minimum, a safe backup plan. The script also adds some basic text (which you can see at the original link I posted). I made a donation on your website as a thank you, I appreciate your help!
From an earlier post in this thread, I see I can style the text using CSS, which I can probably figure out myself.
One more advanced question... would it be possible, instead of just inserting text, to also insert spacers and, say, a footer at the bottom of the page, the location of which is calculated from the screen height? It would also be nice to insert a footer on all store pages. The "footers" across my site are actually simple Markdown boxes with "<h4>©DANCING ASTRONAUT DESIGNS 2022, ALL RIGHTS RESERVED</h4>" in them.