Hello JimAZtec,

Could you be a bit more specific about how this old theme is interferring with your new theme  ? If it's no longer listed on the add-ons page, it should be completely gone .... Could it be that maybe you are using a (custom) Windows theme ?

This was a custom Thunderbird theme to mimic Windows Mail and was installed 4 years ago. It was manually installed from an xpi file.

JimAZtec said

This was a custom Thunderbird theme to mimic Windows Mail and was installed 4 years ago. It was manually installed from an xpi file.

And what makes you think that it's interferring with your new theme ? What happens ?

Same dark colors and layout, no changes when another theme is installed except the status bar.

I'm just going to have to save my data and uninstall/re-install and see if that fixes it.

Just to make sure that you haven't missed a step :

https://support.mozilla.org/en-US/kb/install-themes-change-look-thunderbird

That didn't change anything.

What puzzles me : you remove the old theme; it is no longer listed in the add-ons manager - how could it possibly interfer with your new theme  ?

Could the problem be that new theme  ? Maybe uninstall and re-install like you suggested, would be a good idea .....