Thanks, Monka. No, that didn't help. Basically, I'm trying to put into practice what's shown on this page in the help files. Is there a 'live demo' of the muut-custom element? https://learn.muut.com/advanced-embedding/configuration
I figured out my issues, which have to do with the way Weebly handles things. Short version: the <script src=...> tag has to come at the top of the custom HTML box, and then the CSS reference can be either in there or the page footer code.