Mark, can you unmark this as “Solved”.

Good news is that we’ve upgraded to the latest release - Skuid 10.0 - and I just tried editing our custom theme (both in Incognito Mode and in a regular Chrome Window). In both cases, the page finishes loading without any errors!

For those still struggling, another trick that Mark suggested is that “the Update Out Of Date Themes button will resurface if you import a theme.”

So I imported a copy of our theme and can confirm that I was able to edit the Theme without clicking that button. Full disclosure though: I am hesitant to click that button, as right now, we seem to have Custom Theme Editing working perfectly.

Long-term, Theme Versioning as a feature would be great, so that I could easily rollback the ‘Update Out-of-Date Themes’ button if an error were to occur or a theme became ‘stuck’ after an unsuccessful update.

But for now, we seem to be running smoothly on Skuid 10.0.
Thanks again for all your help Mark & Amy.