Missing theme when viewing local 2024.ploneconf.org site

I decided to try building the GitHub - plone/2024.ploneconf.org: 2024 Plone Conference website site.

After successfully building the back and front ends and starting them and ssh tunneling to ports 8080, 3000, and 3001, when I browse to the front end I see the content on the page but the theme is not rendering.

That was with Safari, even after reloading the page... When I then tried in Firefox the page rendered nicely. And then when I went back to Safari I closed the window and tried in a new one and it worked.

A false alarm, but what would be the explanation?