feat(NM-2213): change sepia bg color

Sources/Navigator/Preferences/Types.swift

@@ -110,7 +110,7 @@ public enum Theme: String, Codable, Hashable {
     private static let nightBackgroundColor = Color(hex: "#000000")!
     // https://github.com/readium/readium-css/blob/master/css/src/modules/ReadiumCSS-sepia_mode.css
     private static let sepiaContentColor = Color(hex: "#121212")!
-    private static let sepiaBackgroundColor = Color(hex: "#faf4e8")!
+    private static let sepiaBackgroundColor = Color(hex: "#f5efdc")!
 }
 
 /// Number of columns displayed in a reflowable document.