Using color-scheme and light-dark() to implement dark mode (#154)

* Lint fixes.

* Implement light-dark().

* Lint css with biome, remove stylelint.

* Biome lint fixes.

* Dark mode adjustments.

* Style bug fixes.

* Colour adjustments.

* Move header and footer styles to base.

* Site header adjustments.

* Removing jQuery and umbrella libs. They where great but not needed anymore.

* Add colorScheme to optionally lock light/dark.

* Update README.

* Biome adjustments.

assets/js/codeblocks.js

@@ -16,7 +16,6 @@
   copybuttons.forEach(function (button) {
     button.addEventListener('click', function (e) {
       const codeblock = e.target.parentElement;
-      const codeid = e.target.dataset.codebutton;
       const codesnippet = codeblock.querySelector('[data-codetext]').getAttribute('content');
       const codearea = codeblock.querySelector('pre');
       codearea.classList.add('flash-item');