I am not a web designer; I'm not a designer of any kind. If you saw me on the street you would never think "now THAT is a fashion savvy gentleman. If i believed that tone-deafness was really a thing I would probably tell you that I have the visual aesthetic equivalent of tone-deafness. So you should feel free to be skeptical of my opinion.
Dark backgrounds with light text is can bad thing. Maybe there are ways to do it effectively I KNOW there are ways to do it and would GUESS it has something to do with the ...amount of contrast between the bg and text.
My main issue I have is the 'burn-in' effect/hallucination/illusion -read a few paragraphs of white (or very light) text on a black background and then jump over to a page with the more popular dark text on light background; you'll likely experience the illusion of 'banding' -what looks like alternating white and dark bands that seem to drift slightly. I'm sure there are technical terms that describe this illusion -and I'd bet that psychologists, ophthalmologists, and/or neurologists could explain the cause of this phenomenon -all to say that my terminology is provably way off but I'm pretty sure that the phenomenon is a common one -common enough that it's worth considering when designing a page. i AVOID sites that cause this effect -or, at least, I groan when a search leads me to a page that uses a very dark background with light text.
It's worth mentioning that I'm not against this because of nostalgia, or because I believe that old ways are the best ways. There's a trend going on now to make dark themes available for UIs -especially n mobile apps. I LIKE this trend and when available I set or configure UIs to darker colours; I often find darker UI elements EASIER on my eyes. It might be relevant that besides being moderately near-sighted, my eyes are 40 years old.
I have noticed that VERY dark blacks (I know, black is black) are often less ...pleasant than matte black or dark grey.
I'm pretty sure that web designers -or anyone who is interested in visual aesthetics of text knows of the 'light text on dark background' issue -I bet there's even a specific term used for this problem. I'm not talking to design professionals or enthusiasts. I'm talking to regular jane or joe who, once in a while, designs a site, page, or blog with function in mind; or to those you who hears crap like "Oh, Jane is really good at computers; let's get her to design our website!". (I feel ya, Jane).
blah, blah, end of monthly rant.
tl;dr: If you're not a designer, be careful of dark backgrounds with light text.