bug#33452: Color of tooltip text on macOS with dark mode

[Tak Kunihiro]

>> When Emacs 26.1.90 is launched on macOS with dark mode, color of
>> tooltip text is set to white. It is hard to read the text.
> 
> Is this problem new with Emacs 26.1.90?

I do not see the problem in 27.0.50.