[web] Standardize spoiler color

Summary: Two things here - the first is that we probably want to make the color of spoilers the same as it is on native, #33332C. The second thing is the current spoiler color on received messages is impossible to differentiate from the message bubble.

Test Plan:
Will link before/after on web

Before	After

Reviewers: atul, tomek, ginsu

Reviewed By: atul, ginsu

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D5836