[web] Introduce spoiler colors in the first section of theme.css

Summary: As mentioned by @tomek in D5836, we should follow convention and introduce a variable in the first section of theme.css and use it instead of hard coding both values.

Test Plan: Ensure that spoilers still appear as before on web.

Reviewers: tomek, ginsu, atul

Reviewed By: tomek

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D5881