[BeckyEbook]

To the settings folder – the same where you found the sigil.ini file – add the custom_preview_style.css file, in which you define your colors, for example:

html {
    background-color: black;
    color: white;
}

These settings only apply to Book View and in the old version of Sigil it doesn't work perfectly, colors can be overwritten by existing styles in the book etc.