Gmail & Outlook — no dark-mode stylesheet. Gmail shows the base design in both modes; classic Outlook for Windows inverts the panel itself in dark mode, the block stays charcoal
Apple Mail — inverts when READ in dark mode; the compose window is always white
Every pane is pinned, so this shows the same thing whatever your own machine is set to. The block stays charcoal in both modes, so the wordmark is never swapped for a second image.