[DiapDealer]

You understand this is strictly for aesthetics, right? We're not going to do a ton of work to make a couple of people feel a little warmer and fuzzier about dark Preview. If being able to easily see all portions of the EPUB equally well is important, then most are simply going to turn off Preview's dark appearance on books where colors and/or borders are important. That's what I do.

You clearly don't understand how Preview's dark appearance is achieved (custom_preview_style.css has nothing to do with it), nor how new gui preferences are added. If you did, you'd understand that it's a rather tedious process that touches a lot files and ends with wildly underwhelming results when all is said and done. Not a single bit of it is "all's ya gotta do is blah, blah, blah and Bob's yer uncle!" None of it is hard. Adding more preferences is just time-consuming and unrewarding. I'd rather spend my time elsewhere. Especially when what you want can already be achieved by adding/editing one special css file (a feature we added for this very reason) in Sigil's prefs directory with no Sigil code changes required. Or by just turning off Dark Preview on books that rely heavily on colors/borders.