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Anyone needing black background and white text simply has a backlight (LCD) or front light (eink) or brightness (OLED) too high, or if eink with no built in light they have a terrible reading light unsuitable for paper books.
The LCD and OLED screens are very different.
LCD uses rear lighting that can be LED, or on old things 18 years ago, CCFL. LCD lighting can be using an array behind a diffuser (big quality TVS) or lighting at the edge of a rear light pipe. LCDs need two polarisers, one front and back and need coloured dye filters as they are monochrome. The quality of the white backlight depends on the LEDs. Only very expensive TVs use Red, Green and Blue LEDs. Older TVs, Laptops and early smartphones used CCFL tubes which can have better phosphors because the tube emits a mix of UVA and UVB. White LEDs do not exist. They are blue, violet or near UV LEDs with mostly a broad spectrum yellow phosphor. They vary hugely in colour temperature and colour rendition.
The various kinds of OLED displays are not real LEDs in the sense of backlights, indicators or ancient calculators. They are electroluminescent dots that are diode like. They usually have phosphors as they don't properly do red and green. They might have filters on the front glass too.
An amber screen filter may work better for LCD and OLED it's too blue at night. The software approach is poor.
So called Dark mode was invented because of people simply having too high a brightness setting on LCD and OLED. It's actually more tiring than white background with proper brightness and ambient light.
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