Add dark mode support

Heavily WIP still, but this begins the process of implementing dark mode for our docs and across the project itself.

- Color modes are toggled in the docs navbar with a custom toggler, which stores the select color mode in local storage.
- Color modes can also be set via data attribute thanks to `data-theme` (with light or dark options available currently).
- Docs are heavily WIP for demonstrating the dark mode.
- In order to best implement color modes, I've spiked out a number of new Sass and CSS variables (e.g., `--bs-secondary-bg` and `--bs-tertiary-bg`). In addition, I've added new global CSS variables like `--bs-border-color` and more. So, in addition to general color modes and theming support, we get greater real-time customization, too.

Todos and open questions:

- [ ] Do we refer to these as themes or color modes?
- [ ] Do we provide a color mode toggler JS plugin?
- [ ] Update all components to better utilize global CSS variables so they can be more easily themed (e.g., see `$dropdown-*` Sass variable changes in the diff).

site/layouts/partials/docs-versions.html

@@ -8,7 +8,12 @@
   </a>
   <ul class="dropdown-menu dropdown-menu-end" aria-labelledby="bd-versions">
     <li><h6 class="dropdown-header">v5 releases</h6></li>
-    <li><a class="dropdown-item current" aria-current="true" href="/docs/{{ .Site.Params.docs_version }}/">Latest ({{ .Site.Params.docs_version }}.x)</a></li>
+    <li>
+      <a class="dropdown-item d-flex align-items-center justify-content-between current" aria-current="true" href="/docs/{{ .Site.Params.docs_version }}/">
+        Latest ({{ .Site.Params.docs_version }}.x)
+        <svg class="bi"><use xlink:href="#check2"></use></svg>
+      </a>
+    </li>
     <li>
       <a class="dropdown-item" href="https://getbootstrap.com/docs/5.1/{{ $group_slug }}/{{ $page_slug }}/">v5.1.3</a>
     </li>