Light-on-dark color scheme
Light-on-dark color scheme, also called dark mode, dark theme or night mode, is a color scheme that uses light-colored text, icons, and graphical user interface elements on a dark background and is often discussed in terms of computer user interface design and web design.
Originally, computer user interfaces were formed on CRTs. The phosphor was normally a very dark color, and lit up brightly when the electron beam hit it, appearing to be green or amber on black, depending on phosphors applied on a monochrome screen. RGB screens continued along a similar vein, using all the beams set to "on" to form white.
With the advent of teletext, research was done into which primary and secondary light colors and combinations worked best for this new medium. Cyan or yellow on black was typically found to be optimal from a palette of black, red, green, yellow, blue, magenta, cyan and white.
While the debate of whether it is easier or healthier to read text on a dark background was disputed by vision and perception researchers, there was similar dispute between users.
A recent article by Popular Science suggests that "Dark mode is easier on the eyes and battery" and displaying white on full brightness uses roughly six times as much power as pure black on a Google Pixel, which has an OLED display. 
In June 2019, Apple announced that a light-on-dark mode will be available across all native applications in iOS 13 and iPadOS. It will also be possible for third-party developers to implement their own dark themes.
Light on dark color schemes require less energy to display on the most common display technologies, including as OLED, CRT and LCD displays. This positively impacts battery life and energy consumption.
While an OLED will consume around 40% of the power of an LCD displaying an image that is primarily black, it can use more than three times as much power to display an image with a white background, such as a document or web site. This can lead to reduced battery life and energy usage, unless a light-on-dark color scheme is used. The long-term reduced power usage may also prolong battery life or the useful life of the display and battery.
The energy savings that can be achieved using Dark mode are because of how OLED screens work: in an OLED screen, each subpixel generates its own light and it only consumes power when generating light. This is in contrast to how an LCD works: in an LCD, subpixels either block or allow light from a permanentely lit LED backlight to pass through.
Issues with the web
Some argue that a color scheme with light text on a dark background is easier to read on the screen, because the lower overall brightness causes less eyestrain. The caveat is that most pages on the web are designed for white backgrounds; GIF and PNG images with a transparency bit instead of alpha channels tend to show up with choppy outlines, as well as causing problems with other graphical elements.
It is not necessary that a web design work well with only one color scheme. There are many mechanisms of web architecture that allow designs to work well with any color scheme a user might prefer. This technical flexibility is a product of the web architect's concern for accessibility and user preference empowerment, though designers rarely utilize this technical flexibility. Wikipedia is an example of this.
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