Hi Philip and David and thanks for the detailed explanations.
Now I understand it all and I've been able to implement my own stylesheet, over-riding the admin-imposed theme. There is a minor drawback in this otherwise perfectly valid workaround: since it is only possible to impose one's own style through a "regular" CSS file, we lose the interesting facility of the "variable color parameters" as originally coded in the styles.php of the themes directories. Which means putting all those variables back to where they belong in one's own CSS file. A little tiresome, but well worth the new customizing freedom it gives me as a teacher.
I'm looking forward to the new "rendering mechanisms" mentioned in David's post (version 2).
Joseph_R