fix(observatory): set sidebar/TOC background-color

[caugner]

Summary

(MP-1318)

Problem

The current item in the sidebar and TOC of the HTTP Observatory docs has no background-color, which is inconsistent with other sidebars and TOCs.

Solution

Define the --background-toc-active variable, which is used for this.

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Screenshots

Before

After

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How did you test this change?

Will deploy to stage by July 5, 2024.