[native] Fix Tooltip color regression

Summary:
Context: https://linear.app/comm/issue/ENG-2259/fix-background-color-of-tooltip-in-dark-mode

This diff reverts the background color of the tooltip to #1F1F1F (which had no previous entry in colors.js) for dark mode.

For light mode I just inverted the color to make sure the contrast was similar-ish.

Test Plan:
Here's how it looks for

Light mode:

Dark mode (identical to before):

Reviewers: ginsu, tomek, rohan, varun, marcin, ashoat

Reviewed By: ginsu, ashoat

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D5666