Discourse Version: 2.6.0.beta1 (232d277833)
Plugin Commit: 76ab7aece83d445c65e185caa852a42283af644b
Steps to Reproduce
Visiting any wizard without a custom theme specified reproduces the issue for me.
You should see that the font has changed to the default font for the browser (DejaVu Serif) in my instance.
The font used to be the font from the Discourse theme.
If you look at the browser inspector in Firefox you will see an error message stating
Style sheet could not be loaded http:localhost:9292/assets/wizard_variables.css
I am in development mode, production sites produce a similar error but with an obfuscated filename.
I don’t have a site in production with the latest CWP. Sorry.