Paint in cyberspace gray

Summary:
We follow M11, based on the palette prepared at
https://agora.nasqueron.org/Design/2017_colors

Ref T1148

Test Plan: Open in browser, check contrast for the 404 page

Reviewers: dereckson

Reviewed By: dereckson

Maniphest Tasks: T1148

Differential Revision: https://devcentral.nasqueron.org/D902