I’ve found two pages explaining how toggling between light and dark modes for Discourse’s embed might be done, but the guy who writes my JS is wondering if the method you’re suggesting entails using postMessage (which the pages I found state) or something else.
Here’s a page explaining how to implement cross-window messaging with postMessage
While this one is actually a tutorial on how to change the CSS within an iframe via postMessage, specifically for switching between light and dark modes:
https://cobwwweb.com/change-css-iframe
Might I be on the right track with all this?