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/_utilities.scss

@@ -538,6 +538,8 @@ $utilities: map-merge(
           "muted": $text-muted,
           "black-50": rgba($black, .5), // deprecated
           "white-50": rgba($white, .5), // deprecated
+          "body-secondary": var(--#{$variable-prefix}secondary-color),
+          "body-tertiary": var(--#{$variable-prefix}tertiary-color),
           "reset": inherit,
         )
       )
@@ -563,7 +565,9 @@ $utilities: map-merge(
       values: map-merge(
         $utilities-bg-colors,
         (
-          "transparent": transparent
+          "transparent": transparent,
+          "body-secondary": rgba(var(--#{$variable-prefix}secondary-bg-rgb), var(--#{$variable-prefix}bg-opacity)),
+          "body-tertiary": rgba(var(--#{$variable-prefix}tertiary-bg-rgb), var(--#{$variable-prefix}bg-opacity)),
         )
       )
     ),