Quick update that I tried editing the theme in Incognito Mode in Chrome, as suggested by Amy Dewaal here: https://community.skuid.com/t/theme-composer-loading-issue-with-brooklyn-iteration-4

The Incognito Mode trick worked once! Upon creating a new theme, the page loaded properly. But as soon as I refreshed the page, the new theme began hanging again.
(also, going into Incognito Mode just to edit a theme seems a little silly. :p)