The browser extension breaks MDN styling

Created by: eight04

• Sourcegraph version: v20.2.7.1403
• Platform information: Firefox Developer v74.0b4

Steps to reproduce:

1. Install the extension
2. Go to https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/API/tabs/onUpdated

Expected behavior:

Shouldn't the extension only modify Github.com?

Actual behavior:

Injected CSS breaks the layout of MDN:

Maybe related https://github.com/sourcegraph/sourcegraph/issues/5889, https://github.com/sourcegraph/sourcegraph/issues/760