(In reply to Mark Striemer [:mstriemer] from comment #0)
This worked pre-proton and is likely happening since we use the system colours, but they aren't set or aren't set to dark colours by the OS. 
That's right - on macOS, CSS system colors currently do not observe system dark mode. But once bug 1623686 is done, they will - you can try this by setting
widget.macos.respect-system-appearance to true locally.
However, while the text correctly turns white with that pref, the background remains white, too! It looks like
-moz-Dialog, which is used for the background, resolves to
rgba(255, 255, 255, 0.098) in dark mode. This is a very low-opacity white color, rather than an opaque dark color. It seems to be intended to be used on top of a dark backdrop.