An update to inform anyone who may also be in need of the Custom Form Styles functionality:

After an hour or so online with customer support where I shared my screen with the support person, I learned this is a known issue. The Custom Form Styles menu option is, in fact, gone from the settings menu for some users. Custom Form Styles can currently be found on the All Lists page, but when you click the link there, only the following error message is displayed:

We're sorry, we can't find the page you requested.

You may have incorrectly typed the address (URL) or clicked on an outdated link.

Again, this is a known issue, and it affects some users but not all.

Hopefully this will get escalated and resolved quickly. I rely heavily on the ability to customize emails as I send out monthly invoices.