Introduce SCSS plugins: dark mode and numbered headings

assets/plugins/_dark.scss

@@ -0,0 +1,9 @@
+$gray-100: rgba(255, 255, 255, 0.1);
+$gray-200: rgba(255, 255, 255, 0.2);
+
+$body-background: #343a40;
+$body-font-color: #e9ecef;
+
+$color-link: #84b2ff;
+$color-visited-link: #b88dff;
+$color-dark-link: $body-font-color;

assets/plugins/_numbered.scss

@@ -0,0 +1,23 @@
+.book-page {
+  .markdown {
+    @for $h from 1 through 6 {
+      > h#{$h} {
+        counter-increment: h#{$h};
+        counter-reset: h#{$h + 1};
+
+        $content: "";
+        @for $n from 1 through $h {
+          $content: $content + 'counter(h#{$n})"."';
+        }
+
+        &::before {
+          content: unquote($content) " ";
+        }
+      }
+    }
+  }
+
+  + .book-toc nav ul {
+    list-style: decimal;
+  }
+}