Your personal preference is not invalid. But think about it this way: making the (very reasonable) assumption that most users who set their system to dark mode

Your personal preference is not invalid. But think about it this way: making the (very reasonable) assumption that most users who set their system to dark mode want their browser content to match, this default behavior does the most good for the most users, even if it isn't what you personally want. I don't think anybody would disagree that a manual toggle makes the user experience even better, but since it's not a part of the browser standard, it isn't zero-cost either. It has to be implemented in code (and incorporated into the layout, etc). Does it get associated with the logged-in user's account? If so the full stack has to be roped in; a new database column may be needed. You could just put it in localStorage, but then if the user switches to another device they could be surprised by the fact that this preference doesn't follow them. It's definitely a nice-to-have, but demanding it as a hard requirement comes across as pretty entitled. A better framing would have been "Hey web devs, there exist people who specifically want their web content to have a different theme from their system, it would be nice to have a manual toggle even if you already support prefers-color-scheme". It's something that never would have even occurred to me, personally, and so this could have been an illuminating post instead of a rant.