puzzling. if folders are white, wouldn't making message content white make them unreadable?

No. In the Dark theme the background is black — and as I said the color of the message titles is yellow. The header of the open message below is again white, but the content of the message is yellow again on black (in text messages). That's OK. What I would like to change is the color of the message list. It seems that they have specified the color in the folder pane but not in the message list pane, and it takes the Windows color as default.

Of course the color of all the messages in the message list pane can be changed with a filter, but I would prefer to use the filters to change the colors of specific messages...