The darkening works fine, but interferes heavily on page loading times. For instance, when trying to create a new event in Google Calendar, this extension adds about 2-5 seconds between the instant I click on an empty bit of calendar and the instant the new event dialog pops up. When I want to click on the X button to close that dialog, I also have to wait a few seconds before my clikc is registered.

I installed the competitor extension Dark Reader and I don't have this problem anymore.