A few things to call out:

1) Dark mode is increasingly not exclusively about the user experience. We're seeing phones switch automatically to dark mode when batteries are low.

2) Dark mode, at least for email, has better readability by design. The dark mode algorithm swaps foreground with background and then adjusts the colors to be at least at a ratio of 4.5 to 1 (wcag 2 compliance). This means you can have unreadable white text over white background in light mode and it will actually be readable in dark mode. So having discussions about whether dark mode is more readable is kind of pointless, it was designed to be more accessible.

edit: If there is interest, I can share a desktop app that helps illustrate dark-mode changes (specifically for email). https://imgur.com/NOGT5el