[web] Set Splash background color to match that of designs

Summary: The background color in the designs is #100E16 (a very deep purple), not black.

Test Plan: Looks as expected (hard to notice change in isolation, but side-by-side it's more evident).

Reviewers: ashoat, tomek

Reviewed By: ashoat

Differential Revision: https://phab.comm.dev/D6546