I am over the worst of my initial cluelessness regarding Sigil, although I have not learned all there is to know and can't encode html's yet. I'll work on that later. At this point, I just need some simple, practical "how do I?" pieces of advice. This will help me to make progress, even if I go in the wrong direction, temporarily, with such advice I got. Mr. Mirkel helped me to find the answer to my question, thanks very much.
The answer is as follows: Right-click on the .xhtml (or .css) you want to use in another Sigil project. Click on "Save as". Give a new name to the file as you save it to an appropriate folder, for instance experiment.xhtml. Now go to the other Sigil project. Right-click on the Text folder icon in the Book Browser (because you are going to paste a .xhtml). Click on "Add existing files". Double-click on experiment.xhtml in the folder where you saved it. This will, in effect, transfer the .xhtml from the first Sigil project to the new Sigil project.
What happens next -- if the pasted .xhtml will work in its new setting -- remains to be seen or worked on.
All I can say is that if you started a Sigil project by opening a writer2epub-produced ePub containing a cover with a background, footnotes, pictures with correct text wrapping, and so on, you should be able to add a .xhtml from another, similar Sigil project without a problem. I guess this is partly because the necessary .css and other auxiliary files are available in the project from the start. Also, if you start a Sigil project by opening a writer2epub-produced ePub, you can develop this extensively within Sigil with good results when you finally compile and read the compilation with your eReader.
I'm working on an ePub with many bells and whistles built up without personally encoding html's, displaying correctly on my Sony eReader. I will post this here when its ready, in case someone might be willing to test it on other devices. In the mean time, I just need some practical help,
