jackie_w, thanks so much for your detailed explanations, the fog is slowly lifting
I have now put all the code in the Extra CSS box in the preferences/common options tab (under ocnversion), and it works!
The only thing which doesn't yet work is the "width: x%", which Calibre refuses to put anywere - but I guess this is a question for the Calibr forum.
Adding the code to Calibre once adn for all makes everything so much easier - and rkomar, thanks for your concern, but the three readers in the house are all pocketbook - and anyhow I now get the drift of how to change things.
Real pity for the lack of a css file for Adobe!