This extension applies the Night Mode by making text color as gray and background color as black. It is useful for reading long time at night.
After completely loading the web page, just hit the icon and it applies the Night Mode on your web page.
If you want to turn it off, just refresh the page you're on to take the things back to normal.
(Note: I have published the extension "Dress-up webpage", which is the next step of this "Night Mode" extension. "Dress-up Webpage" allows you to change the background color, font color, font size and font type. The link to this extension is : "https://addons.opera.com/en/addons/extensions/details/dress-up-webpage/")