You can click the extension icon, and it makes the web page black. Your mouse is a flashlight, and you can use it to see. To turn it off, click the extension icon again.

Changelog:
1.4.1: Fixed an issue with the reset button in the options menu.
1.4: Added a transparency slider and made conflicts with the webpage essentially impossible
1.3.1: Fixed a localization issue in the settings menu.
1.3: Added a settings menu.
1.2.1: Just a really quick bug fix.
1.2: Fixed potential conflicts, added proper touch support, and added localization.
1.1: Fixed issue that caused layout bugs and it to not work on some sites.
1.0: Released.