[web] Fix tool-tip-color color in theme.css

Summary:
Should have been shades-white-60 instead of shades-black-60

Before: https://blob.sh/atul/a999.png
After: https://blob.sh/atul/9a18.png

Figma reference: https://www.figma.com/file/a1nkbWgbgjRlrOY9LVurTz/Comm-%2F-Desktop-app?node-id=1170%3A77305

Test Plan: NA, looks as expected

Reviewers: palys-swm, def-au1t, benschac, varun

Reviewed By: palys-swm

Subscribers: ashoat, Adrian, karol-bisztyga

Differential Revision: https://phabricator.ashoat.com/D3313