What's the Firefox add-on to dim the whole page of a site?

"Do you mean Blackout?

If you are browsing in a dark environment, then it is very bad for your eyes to read pages with white background. This plugin helps you by turning the webpage to a dark color scheme.

There are some predefined color schemes built in, and you can also configure your color preferenes.
The ""Stay in the dark"" function enables you to automatically darken the page when you click on a link and the page loads.
Links and headers gets different color then text."

Another option is to use a global theme with the Stylish firefox add-on. Check out userstylesdotorg for the various styles which are available. There are several 'dark' global styles on the site. If neither the Stylish add-on nor the Blackout add-on seem suitable, it might be worth trying out the 'Dimmer' add-on.