[web] [fix] [ENG-731] update thread selected state text

Summary:
change text color per issue and conversation with ashoat.
https://linear.app/comm/issue/ENG-731/appearance-of-thread-selected-state-makes-it-look-unread

Test Plan: select a chat thread list item on web

Reviewers: atul, palys-swm, def-au1t

Reviewed By: palys-swm

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

Differential Revision: https://phabricator.ashoat.com/D3698