Hmm, I haven't heard about theme changes before.

The Add-ons team is working on a fix for the "can't be verified" issue. I don't know whether that will involve running an add-on update or something else. We're all hoping to learn soon.

M.ozilla said

Title says it all. My Add-ons have been removed and the browser mysteriously converted itself into the dark theme found in the customization tab though it is selected to default.

There was a Bug-A-Boo with a Certificate Expiration Date dealing with the Add-ons. There's a new FF Version with the main Fix, 66.0.4.

Mozilla Blog Post Update:
A Firefox release has been pushed — version 66.0.4 on Desktop and Android, and version 60.6.2 for ESR. This release repairs the certificate chain to re-enable web extensions, themes, search engines, and language packs that had been disabled (Bug 1549061).
There are remaining issues that we are actively working to resolve, but we wanted to get this fix out before Monday to lessen the impact of disabled add-ons before the start of the week. More information about the remaining issues can be found by clicking on the links to the release notes above.
(May 5, 16:25 EDT)

~Pj