[web] Inherit colors on spoiler reveal animation

Summary:
Solution for ENG-4675.

This diff fixes the issue where, after the spoiler reveal animation, the content color was being changed.

Depends on D8877.

Test Plan:

1. Before applying this patch, check if current spoiler reveal animation does change the content color:

2. Apply this patch and see if colors are not changed:

Reviewers: tomek, inka, rohan

Reviewed By: inka, rohan

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D8876