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.
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.
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.)
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?
Dark Mode should only be for UI elements not for actual content. They would never do that with a PDF either.