Dear Michal,

aaah, I see. In fact, my problems were seen with TikZpictures, i.e. I'm the same guy from here:
https://tex.stackexchange.com/a/641354/224985

The black pgfornaments became invisible with a dark theme for me by default, which confused me at first. I then tried with a colourful TikZ picture and saw the colours became inverted, i.e. I used:

\tikzset{pgfornamentstyle/.style={color=black,fill=yellow, opacity=0.7, line width=0.5pt}};
\node[anchor=south]{\pgfornament[height=1.0cm]{87}};

within that example and the ornaments came up with a blue fill colour. But you are fully correct, it does not happen to \includegraphics.

The CSS config you added works perfectly fine for me in the case of black TikZ images, it makes them visible nicely and the body and text are well adapted to the dark theme. So I think this is a large improvement to the default :-).

Dear Karl,

feel free to close if this is intended — after updating from TeXlive 2021 to 2023, I noticed that with the default CSS and HTML5 mode in a dark-themed browser, all image colours are inverted.

Body background colour and text colours are unaffected, but images are caught by the rule:

\Css{
@media (prefers-color-scheme: dark) {
img[src^="\jobname"]{filter: invert(1); }
}
}

which was added in https://tug.org/svn/texlive/trunk/Master/texmf-dist/source/generic/tex4ht/tex4ht-html5.tex?r1=64099&r2=65513 . The comment there mentions this is a fix for:
"background of generated images in the dark mode"
so I presume it was added on purpose.

It seems quite irritating for a default — black graphics (e.g. pgfornaments) vanish against the white background when shown in a dark-themed browser, and photos get inverted colours.

I'm personally using:

filter: drop-shadow(7px 7px 4px rgba(0, 0, 0, .7)) invert(0) !important;

in my CSS for all images to overwrite that. and of course "nostyle" might be what I should actually be using when using a custom CSS.

As outlined, if the default is intended, feel free to close.
Thanks!