[web] Implement darker thread color tint to blockquote container

Summary:
Linear issue: ENG-1712. Restyling the quotes by layering the blockquote with a low opacity black background to create the darker tint effect

Next Steps:

1. Update non-viewer blockquote message container color to be the base thread color

Test Plan:
Please view the attached screenshots to see the target design in the figma document and the before and after of the changes I implemented

Figma document: Figma.

Before:

After:

Reviewers: atul, ashoat, kamil

Reviewed By: atul, ashoat, kamil

Subscribers: kamil, jacek, ashoat, tomek, atul, abosh

Differential Revision: https://phab.comm.dev/D4999