In Thunderbird 78 you can find the settings / options in the "Tools" menu → "Add-on Options".
This is the result of the technology used (WindowListener API) in the associated Add-on version. The Add-on behaves exactly as one was "used" to do with Thunderbird 68. Incidentally, this applies to all Add-ons that have been made compatible with Thunderbird 78 using the WindowListener API.
From up Thunderbird 91 will the options of this (and all other) add-on(s) be accessible via the Add-on Manager, which is the result of the new MailExtension API. In Thunderbird 78 there will be a "mix" in Tools menu and in Add-on Manager.
QuoteColors could look like this in Thunderbird 91 in Add-ons Manager options:
https://www.thunderbird-mail.de/attachment/155…-clipboard-png/