[landing] Fix landing background

Summary:
I broke the landing page's background in D6003 by adding a CSS selector that applied globally to html and body.

Turns out doing it right is a bit harder. In order to avoid needing dynamic imports, I needed to stop relying on html and body for styling, as they appear above #react-root. To make that happen, I had to move the background styling to inner divs, which required moving some CSS attributes around to get everything to look right.

This solves https://linear.app/comm/issue/ENG-2535/landing-page-background-is-messed-up

Test Plan: Make sure http://localhost:3000/commlanding/ has the proper background (both layers of CSS background, plus the stars, and with scrolling looking right) and that http://localhost:3000/commlanding/siwe just has a gray background

Reviewers: atul, ginsu, tomek, jon

Reviewed By: atul, ginsu

Differential Revision: https://phab.comm.dev/D6015