Introduce SCSS plugins: dark mode and numbered headings

exampleSite/assets/_custom.scss

@@ -1,2 +1,3 @@
-@import "variables";
 // You can add custom styles here.
+
+// @import "plugins/numbered";

exampleSite/assets/_variables.scss

@@ -1,13 +1,3 @@
-// You can override SASS variables here. Below example of simple dark theme.
+/* You can override SASS variables here. Below example of simple dark theme. */
 
-/*
-$gray-100: rgba(255, 255, 255, 0.1);
-$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;
-*/
+// @import "plugins/dark";

exampleSite/content/docs/more-examples/advanced.md

@@ -49,4 +49,4 @@ nec, nate, ignari, vernum cohaesit sequitur. Vel **mitis temploque** vocatus,
 inque alis, *oculos nomen* non silvis corpore coniunx ne displicet illa.
 Crescunt non unus, vidit visa quantum inmiti flumina mortis facto sic: undique a
 alios vincula sunt iactata abdita! Suspenderat ego fuit tendit: luna, ante urbem
-Propoetides **parte**.
+Propoetides **parte**.

exampleSite/resources/_gen/assets/scss/book.scss_48b060fe05b0a273d182ef83c0605941.json

@@ -1 +0,0 @@
-{"Target":"book.min.e2750c58bf0545d36d2426996f9d6108a275d000f33e7552d83ea28bdcd34ab7.css","MediaType":"text/css","Data":{"Integrity":"sha256-4nUMWL8FRdNtJCaZb51hCKJ10ADzPnVS2D6ii9zTSrc="}}

exampleSite/resources/_gen/assets/scss/theme/hugo-book/book.scss_50fc8c04e12a2f59027287995557ceff.json

@@ -0,0 +1 @@
+{"Target":"book.min.4e5c3d92718456dab80c87ced0c4c63d3984fb3a742f718957ca25c01c6dcdb6.css","MediaType":"text/css","Data":{"Integrity":"sha256-Tlw9knGEVtq4DIfO0MTGPTmE+zp0L3GJV8olwBxtzbY="}}