Fix syntax highlighting and click-to-copy buttons for code blocks (#2049)

changelogs/internal/newsfragments/2049.clarification

@@ -0,0 +1 @@
+Fix syntax highlighting and click-to-copy buttons for code blocks by purging less CSS.

postcss.config.js

@@ -1,11 +1,21 @@
 // Remove unused CSS selectors.
 const purgecss = require('@fullhuman/postcss-purgecss')({
-    // Use stats generated by Hugo.
-    content: [ './hugo_stats.json' ],
-    defaultExtractor: (content) => {
-        let els = JSON.parse(content).htmlElements;
-        return els.tags.concat(els.classes, els.ids);
-    }
+    content: [
+        // Use stats generated by Hugo from HTML content.
+        './hugo_stats.json',
+        // Add used JS scripts.
+        process.env.HUGO_PUBLISHDIR + '/js/click-to-copy.min.*.js',
+        process.env.HUGO_PUBLISHDIR + '/js/main.min.*.js',
+    ],
+    extractors: [
+        {
+            extractor: (content) => {
+                let els = JSON.parse(content).htmlElements;
+                return els.tags.concat(els.classes, els.ids);
+            },
+            extensions: ["json"],
+        },
+    ],
 });
 
 module.exports = {