This reply is much appreciated Mark, and thank you for pointing me to Layouts, which I hadn’t paid much attention to before. It looks as though that is intended for saving the layout of the working environment, whereas what I’m thinking of has more to do with with how text is displayed when composing.
I was also skeptical of the idea of using themes and templates that other people had put together, but I found that the Ulysses Style Exchange offered the opportunity to quickly try out various color schemes and so forth until I found one that suited me. I would never have thought of setting up my own writing environment to use white text on a dark blue background, but I am finding that works better for me than the classic black text on a white background. I have also found that switching to a monospaced font (Cousine) has made composing text easier for me. But as you say, to each their own!