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/_default/docs.html

@@ -1,5 +1,5 @@
 {{ define "main" }}
-  <div class="container-xxl bd-gutter mt-3 my-md-4 bd-layout">
+  <div class="container bd-gutter mt-3 my-md-4 bd-layout">
     <aside class="bd-sidebar">
       <div class="offcanvas-lg offcanvas-start" id="bdSidebar" aria-labelledby="bdSidebarOffcanvasLabel">
         <div class="offcanvas-header border-bottom">