Ok, I think I figured this out. Calibre merges extra_css into stylesheet.css and also renames it, e.g. style=''calibre7", that's why I didn't find my styles so easily.
I must also research what happens if I redefine the style (does my style replace the old one or a merge is done?) and how no_stylesheets influences these procedures. Calibre sometimes keeps the names of the original styles (sanitized, e.g. without underscores _), sometimes not.
Is there a chapter in manual about this, how CSS is handled exactly?