[DiapDealer]

I'm not sure I follow. There is no "user color" for Preview. There is the book's css. The only thing dark appearance in Preview does is attempt to inject (via css) text/background colors. i.e. white text with the current Window color for background. And it only works if the ebook hasn't already explictly set text/backgound colors via css.

I'm just not certain we could offer anything "inverted", since we have no way of knowing what theming (either with style plugins on Linux, or custom css on all platforms) is already in play. The current Window background color and light text is only a compromise. One that I'm not sure we can (or should) offer any alternatives for. It's already very fragile as it is. That's why it's not enabled by default when Sigil is using a dark theme. The custom preview css feature would be better suited for those who wish to customize their dark-mode (or any mode for that matter) Preview colors in my opinion.

Remember that the inherent dark mode you're likely forcing on Sigil (with the environment variable) on Linux is only one of many dark themes that are available to Linux users via custom Qt theming engines and Qt style plugins. We use the background color of the current theme and light text to approximately match everything. And as mentioned, it only works when ebooks don't force their own text/background colors.