This plugin works great. I think adding a new "toggle domain (in the list)" button would be a good idea.

Darkening effects are very good, but the white flash at the start of page loading is unpleasant. Looks like the loading speed of the add-on is a bit slower.
Also, instead of a simple click on the "Add" button to add the current domain to the exclusion list, I have to manually input the domain to the text box before clicking the "Add" button, which is not very convenient.
P.S. It would be great if the add-on can apply darkening effect based on the color of the page (some pages are already dark and should not be inverted)