Here's a feature request, a function to edit or remove text color settings in the stylesheet.css file. I spent a lot of time trying different converters from epub to Mobi but none of them would get rid of what had the text locked to black (actually a really really dark grey). Finally, in response to a post in the epub forum, I was pointed to Sigil and looking for references to color in stylesheet.css. I found three instances of color: #231F20; deleted them and saved. Then the book converted without the text color locked.
I figure the best setup would be to find each color setting and show an example of what changing or removing each setting will do. Might not want to remove *all* the color settings, in this book the TOC links and chapter headings are set to "blue" while the rest of the text was 231F20. I left the "blue" and removed the others.
Boggles me why anyone would set a color to look like printed "black" ink when black text is the default on readers and reader software.
I read on my LED screen phone in white on black because it improves the battery life a bunch VS black on white. Black pixels on LED screens use zero power, VS white pixels on LCD using zero power, other than the backlight shining through. No backlight on LED.