Change dark theme from black to dark gray and add a black theme

cps/static/css/epub_themes.css

@@ -4,11 +4,16 @@
 }
 
 .darkTheme {
-	background: #000;
+	background: #202124;
 	color: #fff
 }
 
 .sepiaTheme {
 	background: #ece1ca;
 	color: #000;
+}
+
+.blackTheme {
+	background: #000;
+	color: #fff
 }