@BetterRed...Have a look at the thumbnail of my Sigil app with dark theme below. It's nice but the dark blue text color for the top htm xml headers can barely be seen in the main tab pane in HTML View. I think it looks OK apart from that and the fact that Text View is almost unreadable and one or two other issues. I'm going to try changing the background color of the main tab pane in the "qt_styles.qss" stylesheet to a blue(like the old Norton Turbo C app) for a better contrast effect.
I've moved this chat topic to a new topic thread in the Sigil blog, with instructions on how to install it, in case anyone else wants to try a dark theme in their Sigil app. The link to the new topic is here:
Adding A Dark theme to Sigil