In the days of cathode ray tube computer monitors it was considered wise to have a dark background (in Windows vernacular: wallpaper). A dark background reduced the intensity of the electron gun or guns illuminating the screen. Reduced intensity also reduced phosphor degradation. Screen savers were more-or-less required to help increase gun and tube life.
Is the same thing true for modern flat screen technology? There are no electron guns nor phosphor. But does a white screen degrade the monitor more than a dark screen?
It depends on the screen type. LCDs don’t really have a problem with this. They function more as filters for a backlight. The backlight’s intensity may degrade over time, but that’s the same regardless of the color on screen. It is possible for IPS LCDs to get temporary “burn in”, but that fades over time.
However, OLED pixels (not “LED”, which is just misleading name for LCD screens) do wear out over time, which can cause burn-in. And the brighter the pixel, the more it wears out. People are generally cautioned not to turn up the brightness more than necessary to extend their lives, to let them go to sleep when not needed, and to use the built in features that can minimize the issue by moving static images around slightly. And OLED monitors usually come with a program that will attempt to even out the wear.
That said, they still last a long time. I have a Pixel 3 XL, which came out in 2018, and the screen is still perfect, with no sign of any burn-in. If there is any loss of brightness, it is not noticeable. That said, people who use the large OLEDs as TV/movie/computer screens and leave them always on often seem to have problems a couple years later.
Fortunately, there is a new synthetic microLED technology that should reduce or even eliminate the wear-out problem with OLEDs, which use organic compounds (hence the name).
Bright screen backgrounds may not be a problem with modern monitors, but they’re still hard on the eyes, especially if you work on a computer all day. I set things to dark mode whenever possible, including using the "Straight Dope Dark’ theme.
I mostly just recommend turning down the brightness. Dark mode is a nice thing, but it’s not really a substitute for getting the brightness of your screen at a comfortable level, IMHO, and using good lighting wherever you are. If you don’t, you’ll still have those white letters which can then leave after images in your eyes.