[web] [fix] update chevron color

Summary:
chevron color should be shades-of-white-60 per figma in the thread ancestor settings on the chat screen.

https://www.figma.com/file/a1nkbWgbgjRlrOY9LVurTz/?node-id=1170%3A77104

Test Plan: double check figma that this is correct

Reviewers: atul, ashoat

Reviewed By: atul, ashoat

Subscribers: ashoat, palys-swm, Adrian, karol-bisztyga

Differential Revision: https://phabricator.ashoat.com/D3327