[web] [fix] hover state background color on thread list

Summary:
https://linear.app/comm/issue/ENG-869/thread-list-hover-background-is-wrong-color, update the hover color

before:

Test Plan: hover color should match figma

Reviewers: atul, ashoat

Reviewed By: atul, ashoat

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

Differential Revision: https://phabricator.ashoat.com/D3421