calibre-web/cps/static/css/epub_themes.css

.lightTheme {
	background: #fff;
	color: #000;
}

.darkTheme {
	background: #202124;
	color: #fff
}

.sepiaTheme {
	background: #ece1ca;
	color: #000;
}

.blackTheme {
	background: #000;
	color: #fff
}