Ok, I tried that, my test epub was
Verne's Off on a Comet
I tried adding the following to the css:
.calibre {
-webkit-transform: scale(-1,1);
background: #faebd0;
background-color: white;
color: black;
display: block;
font-size: 1em;
text-align: justify;
width: auto;
padding: 0;
margin: 0 5pt;
border: currentColor none 0
}
(That is an exact copy of the .calibre entry in the .css file after converting the book with all the presets. I just added the first line and tried again with the original file)
It completely failed; no matter whether I pointed to a new .css file with that as an entry or had it as raw css, the output file was the same and no transform took place. I am perplexed because that seemed an obvious and easy answer.