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/content/docs/5.1/components/dropdowns.md

@@ -998,9 +998,13 @@ As part of Bootstrap's evolving CSS variables approach, dropdowns now use local
 
 {{< scss-docs name="dropdown-css-vars" file="scss/_dropdown.scss" >}}
 
-Customization through CSS variables can be seen on the `.dropdown-menu-dark` class where we override specific values without adding duplicate CSS selectors.
+{{< callout info >}}
+Dropdown items include at least one variable that is not set on `.dropdown`. This allows you to provide a new value while Bootstrap defaults to a fallback value.
 
-{{< scss-docs name="dropdown-dark-css-vars" file="scss/_dropdown.scss" >}}
+- `--bs-dropdown-item-border-radius`
+{{< /callout >}}
+
+Customization through CSS variables can be seen on the `.dropdown-menu-dark` class where we override specific values without adding duplicate CSS selectors.
 
 ### Sass variables