Hi Matrim. Cauthon,
Thanks for choosing Microsoft Community.
Based on your description, I know your requirement. So sorry to bring you convenience. First, you need to make sure you are using Outlook for Microsoft 365, if you use Outlook 2016 ,outlook 2019 or outlook 2021 and so on. As you can see through the document ,this feature cannot be used. Your understanding is highly appreciated. Here is the reference: Dark Mode in Outlook (microsoft.com).

If you use Outlook for Microsoft365, I did a test for you. Through my test, it seems the option is still persists.

So please be kindly to choose another Theme (such as colorful) then click Ok. After it, open file and choose black Theme again to check if the option will show up.
If it does not appear, please make sure your outlook is update to date. If not, you can update your outlook and check the result.
If there is any update, please feel free to let me know.
Thank you so much for your precious time. Stay safe and healthy.
Have a nice day.
Best Regards,
Katherine