[landing] update landing background color

Summary:
This diff addresses the second step for implementing the redesign of the blackish landing page background.

Figma
Depends on D7757

Test Plan:
Here are some screenshots of the landing page which show the new background for the redesign

Reviewers: atul, kamil

Reviewed By: atul

Subscribers: ashoat, tomek, atul

Differential Revision: https://phab.comm.dev/D7759