[web] [refactor] css color should be white

Summary:
color is white, css class should be white. I think this was left over from the theme'ing dark mode changes. More of the thread list item will be updated in coming diffs

Test Plan: color in message preview should still be white

Reviewers: atul, def-au1t, palys-swm

Reviewed By: palys-swm

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

Differential Revision: https://phabricator.ashoat.com/D3500