I share the opinions of GrandFather and Tux.

I don't get why people want black backgrounds. The last time I payed attention, it was all about energy saving, and at that time, when most people stared at LCD, it was utterly nonsense:

Back in the old days, when I still had my old ZX Spectrum+, having green text on black background felt cool, like working on an even older terminal (DEC VT100 and the like). Even when I was only playing with the Sinclair BASIC interpreter. Back then, CRTs were the dominant display technology, having a black background with only a few bright pixels for the text really had advantages. You would use a little bit less energy (black pixel = no electron beam = no current = no power needed for the pixel). That was not that important for the environment or the electricity bill, but it also reduce the load on the high voltage generated (as a by-product) by the horizontal deflection transformer. Stabilisation of the high voltage was not that great for decades, and so the numnber of bright pixels per line influenced the horizontal deflection. Bright lines needed more energy, so there was less energy left for the horizontal deflection, and so the image width was reduced for the bright lines. Keeping the background black largely avoided this effect. On later CRT monitors (and TV sets), high voltage stabilisation was way improved, and so the advantage of a black background for the image stability was gone.

I still own an Eizo T566 CRT monitor (17 inch 4:3). Its technical data report a power consumption of 110 W. I don't know the exact anode voltage of the CRT, but it is usually around 25 kV, a little bit less on smaller CRTs, a little bit more on larger CRTs. The anode current for a white image is typically around 1 mA to 1.5 mA, a little bit more on Sony Trinitron CRTs, a little less on CRTs designed to run 24/7. Multiplying both values gives about 25 W to 40 W used to display a white image. In other words, switching from 100 % white to 100 % black reduces the power consumption by 25 W to 40 W, about a quarter to a third of the total consumption.

Now, LCDs in several variants are standard, the older ones used cold cathode backlights, newer ones use LED backlights. The backlight, as the name implies, illuminates the backside of the LCD. Typically, this is implemented as a large diffuser illuminated from one or both sides(!) of the screen. The LCD is switched to block light for a black pixel, and to let light pass for a white pixel. What you see on a (monochrome) LCD are shadows in front of a constantly bright backlight. (Color LCDs have R/G/B color filters the optical path, so you get colored shadows.) Switching an LCD pixel needs power, but a steady-state pixel does not. The main consumer for a static image is the backlight, and because its brightness does not change with the image, the power consumption is also constant. Or shorter, it does not matter if an LCD shows a dark or a bright image.

Modern LCDs have backlights splitted into a few rectangles that can be individually dimmed depending on the image content. This is nice if the image is a dark szene with only a few bright spots. But if you fill the screen with white text on a black background, the entire backlight has to be as bright as the brightest pixel. So, no energy savings here.

Displays based on a matrix of tiny LEDs (in contrast to LED-backlit LCDs) are rare and expensive, especially at sizes typical for monitors and TVs. (Displays used in statiums may use LEDs, but with pixel sizes in the mm range.) Like CRTs, their energy consumption depends largely on the bightness of the image. A black image simply has all LEDs switched off, so no current. LEDs may burn in, very much like phospor-based CRTs. In order to equalize wear on the LEDs, modern LED-based TVs and monitors turn on the less-used LEDs in standby mode and intentionally wear them to to reach the wear level of the LEDs that were bright while the TV/monitor was turned on. So again, no energy saved.

Alexander