Hm, I added, as you advised me, in Preferences under Common options, an @page CSS under Extra CSS like that:
Code:
@page
{
margin: 5pt
}
and chose "apply".
I closed Calibre and re-started it. Then, I chose a sample epub for conversion, containing (before conversion) only the @page rule for bottom and top. At conversion, under "Layout", my extra css did not appear. Conversion had no effect on the @page as desired. Do I have to always fill in my css style again?