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).

scss/_dropdown.scss

@@ -1,3 +1,5 @@
+// stylelint-disable custom-property-empty-line-before
+
 // The dropdown wrapper (`<div>`)
 .dropup,
 .dropend,
@@ -171,6 +173,7 @@
   white-space: nowrap; // prevent links from randomly breaking onto new lines
   background-color: transparent; // For `<button>`s
   border: 0; // For `<button>`s
+  border-radius: var(--#{$variable-prefix}dropdown-item-border-radius, 0); // stylelint-disable-line property-disallowed-list
 
   // Prevent dropdown overflow if there's no padding
   // See https://github.com/twbs/bootstrap/pull/27703