I understand a dark theme has been suggested before and may not be high on the priority list, which ordinarily is an understandable dismissal of a new feature. It should, however, be considered more seriously for a number of reasons. OLED screens on mobile devices have become increasingly more common in modern premium devices. Due to the way OLED screens are designed and manufactured, devices equipped with them benefit significantly when it comes to darker or black images, not unlike the typical black background in nearly every single Khan Academy video I've seen so far.
An OLED panel is typically composed of four primary layers: The substrate, which acts as the structural framework; the anode, which draws electrons; the cathode, which provides electrons; and the organic layer between. That organic layer is further divided into a conducting layer—which provides the "electron holes" that the electrons flowing through layer can snap into, shedding energy in the process—and an emissive layer where the light is actually produced. And if you want to start messing with producing actual color, it's just a matter of adding red-, green-, and blue-tinted plastic layers to the substrate.
When a device only needs to display a static pattern with relatively slow refreshes you can use something called a passive matrix OLED (or a PMOLED). These work by turning on voltage to specific areas of the film and leaving them on until the device refreshes its instructions. Then there's active matrix OLEDs, like the AMOLEDs you might find in a smart phone. These are for high-definition applications that demand fast refresh rates, such as smartphone screens, tablets, or HD televisions. AMOLED displays require a thin film transistor back-plane to actually drive each of the individual pixels, but this layer is just as flexible as the others, allowing for the development of rollable, foldable, transparent display panel prototypes.
Why is this important?
The LEDs in today's LED televisions are actually used only to provide a white back light, which then shines through a rapidly-refreshing LCD shutter array which tints the emanating light. OLEDs, on the other hand, operate as both light source and color array simultaneously.
An OLED display doesn't need any of the electronics and circuitry used to drive the LED back light and LCD shutter from a LED display, which makes OLEDs more efficient. LED screens produce black simply by fully closing the pixel shutter—the back light is still shining (it never actually turns off) but the light itself is being blocked. An OLED instead turns the pixel off entirely to produce the color black, saving energy in the process. Since OLEDs incorporate their own color filters, they can produce deeper blacks and a wider gamut array. The lack of a permanently-on backlight promotes higher contrast ratios (the difference between the brightest and darkest pixels on the screen).
With the typical background that is in most Khan Academy videos, this alone would save a tremendous amount of power consumption while watching your videos. If used throughout both the mobile app and throughout the website, any viewer with an OLED screen would passively benefit from significantly less power consumption, since Wi-Fi and the screen use up such a huge amount of a mobile devices power. This benefit is only from #000000 hexadecimal value or rgb(0,0,0) in RGB color code.
This benefit goes further than just power consumption as well, as a bit of a developer myself, with sensitive blue eyes living in southwest Florida which is consistently among the highest UV index in the country (apart from a few Midwest states) according to https://www.epa.gov/sunsafety/sun-safety-monthly-average-uv-index#tab-1
I tend to be a bit of a Vampire and rarely go outside more than I need to. I have grown to hate the light as it is legitimately harmful to my eyes down here. I spend a great deal of time in the dark and when I view your site or app I am blinded by bright white light after my eyes adjust to the darkness. I will be blinded by white light and then your videos will have a black background and then the video will typically be long enough for my eyes to begin adjusting to the lack of light and then I'm blinded again by the white background again.
A black or dark background would be far easier on the eyes given how your videos already are made. This setting could easily be optional throughout the entire site or app and would be completely user preference.
If I could have a dark theme on every app, I would and any app that offers it, I enable it immediately.
There are several benefits to a true black theme and would allow for more nighttime or low light environment users as the lack of blue light is what is said to be a contributing factor of stimulation before one goes to sleep. I feel that a dark theme should be thought about more seriously and I feel now it is much easier to understand why.
Thank you for your time,