[web] [fix] border color to match figma design

Summary:
match figma design (https://www.figma.com/file/a1nkbWgbgjRlrOY9LVurTz/Comm-%2F-Desktop-app?node-id=1170%3A79456)

before:

after:

linear: https://linear.app/comm/issue/ENG-798/border-color-in-layout-should-be-404040

Test Plan: check hex value, make sure they match.

Reviewers: atul, palys-swm

Reviewed By: palys-swm

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

Differential Revision: https://phabricator.ashoat.com/D3303