bug#33452: Color of tooltip text on macOS with mark 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.

 Switch to dark-mode on macOS
 $ cd ~/emacs-26.1.90/
 $ ./configure; make install
 $ ~/emacs-26.1.90/nextstep/Emacs.app/Contents/MacOS/Emacs -Q
 M-: (setq mouse-drag-and-drop-region 'meta)

During dragging region, tooltip shows text in the region.  Color of
tooltip background is yellow, and color of tooltip text is white.

I think that color of either tooltip text or tooltip background should
be darker.