I just went to addons and themes. Because I really only use whatever Thunderbird defaults are. In the recommendations I selected Thunderbird suave and clicked install. It installed and did not make everything dark. So I am guessing it is something related to your unnamed theme. No name means I can't try it out for you.

Thank you for the response. I have tried it with multiple themes and it doesn't work right (Genesis, Abstract Space Blue, Circular Logic, Blue-FK, and several others). I tried updating the themes and there were no updates. I just installed another one as a test (The Galaxy of Andromeda) and it does the same thing: theme is at the top of the screen (correct), the side and middle are Dark Mode (wrong), and the main area (where it shows the contents of an email) is normal.

Why not try the one I tried. Then we might have common results to figure from.

You might want to try toggling the setting for "use hardware acceleration" in settings. Just to see if the video driver is playing a part.

I did try that theme and many others, as well as trying to toggle hardware acceleration (there is no setting for that on my system, possibly because it's a notebook PC with on-board Intel and Nvidia graphics chips). Anyways, it seems that this is just how Thunderbird 102.2.2 handles themes, and most themes seem to use a "Dark Mode" instead of a "Light Mode."