Welcome to Meta 
I would join the @theme_creator group, which allows you to share a theme on theme creator so you can look at it without logging in.
You could also add your testing theme as an available theme for users on your forum (but not the new default). With the Sidebar Theme Toggle you could select that theme while you are logged out. But users of course could still see it.
A dev install might also be easier than you expect. I started with a codespace on GitHub where I installed my themes.
Signing up for a 14-day free Discourse trial and using that for testing would work too.