Improvements

- Edit code colors and add a color for comments
- Add ::selection
- Small edits for cards: last-child and overflow
- Add card-sectioning (header, footer, pre)
- Change global hue for Primary color
- Remove border for pre
- Add querySelectorAll() for theme-switcher.js
- Update docs

scss/content/_typography.scss

@@ -309,3 +309,8 @@ ins {
 del {
   color: var(--invalid);
 }
+
+// selection
+::selection {
+  background-color: var(--primary-focus);
+}