readded hugo-book theme files.

themes/hugo-book/assets/book.scss

@@ -0,0 +1,19 @@
+@import "defaults";
+@import "variables";
+
+@import "normalize";
+@import "utils";
+@import "main";
+@import "fonts";
+@import "print";
+
+@import "markdown";
+@import "shortcodes";
+
+// Custom defined styles, theme mixins also placed there
+@import "custom";
+
+// Select theme mixing based on site parameter
+:root {
+  @include theme-{{ default "light" .Site.Params.BookTheme }};
+}