Even if you don't have a dark theme already, it's a huge value-add..... (@dang?)
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If I have to adjust my monitor brightness to be okay with reading something there's a major issue.
Yes, but the issue is with your monitor brightness or your ambient lighting, not the content. I tune my monitor for use at night with the lights on and as a result, #000 on #FFF is perfectly comfortable at all times. There's rarely a good reason to use a PC in complete darkness and I can't think of any reasons to do it in your own home.
I spend way too many hours using a monitor to let it's lifecycle determine what color-scheme I'll spend those hours looking at.
That's 3 pretty big assumptions :)
Sure, in that situation light mode is great, but if I'm browsing HN on my laptop at night in bed in the dark, I would definitely prefer dark mode.
I think that's the key point there. Light themes are good in light, but bad in dark situations, and vice versa. "Most" people prefer light, because they use their computer mostly in the light. Many programmers etc. prefer dark because they use their computers late in the dark a lot.
These are basically two "modes" of use, but software has been treating this as a preference, rather than a mode switch.
Through a set up of PowerShell/Bash scripts, I now have global dark/light mode switches on all my devices and I feel like this is the first time I can really use them effectively in all situations. Light when there's light, dark when there isn't. I even switch on the train as it goes through a tunnel. IMO, this is how it should be used, not as a set-and-forget preference.