I've just submitted v2.3.6 of One Page Checkout for the Zen Cart moderators' review; I'll post back here when it's available for download.

This release contains changes associated with the following GitHub issues:

#278: Remove unneeded create_account.css files.
#279: Correct CSS validation issues for the checkout_one.css files.
#280: Correct "Guest checkout can't complete when country/state dropdowns are used". This bug was introduced in v2.3.5.