Curious, I ended up turning off dark mode because I found web pages were jarringly bright compared to everything else on the screen.

Don't you use websites that have a dark theme more instead. I always preferred websites that allowed me to select a dark theme and it seems to be more and more popular to do that.

Hell even you-tube does that now.

This is great, it means many sites won't have much work to support the CSS media query.

I still would prefer to change a setting once, rather than establish a profile on every website to enable a dark theme.

Well, Youtube should have had a dark theme from day one. It makes zero sense to show videos on a bright white background.

It's really a matter of preference. I personally would be happy if my PDFs would automatically display in "dark mode" (not that I expect that to be implemented). Perhaps browsers just need a setting to toggle this.

Ctrl+I on Evince/Atril doc viewers. Then set as default: Ctrl+T

On Linux I use MuPDF which can also invert. (Although this isn't really "dark mode", since most PDF documents are black text on white, it gets the job done.)

Moon reader + on android allows you to invert the colours and that's what I do every time when I read pdf's and the like.

It is nice to have the option of map pdf colors to correspond to one's color theme.

I like to read pdfs in emacs alongside my notes, and I am very pleased that emacs' pdf-tools allows me to map colors as I choose (kind of - there are only two colors, foreground and background).

Why not? I have a chrome extension to darken and/or invert webpages and PDFs, and I have SumatraPDF setup white text on black background for editing LaTeX?