Add dark mode support (#35857)

* Add dark mode to docs

* Minor fix: missing space indentation

* Minor fix: revert utilities/z-index added-in modification

* Remove prev: and next: from doc because extracted to another PR

* Use .bg-body-tertiary in all Utilities > Overflow examples

* fix example

* Fix up spacing examples

* Update box-shadow Sass variables and utilities to auto-adjust to color modes

* Remove unused docs class

* Refactor form styles to use CSS variable for background images on .form-check and .form-switch

* Fix docs selector

* Rename shortcut for clarity

* Heading consistency

* Reintroduce missing 4th grid item in Utilities > Spacing example

* Fix bundlewatch

* .bd-callout* rendering is OK so removing comments in the code

* Update scss/_utilities.scss

Co-authored-by: Julien Déramond <julien.deramond@orange.com>

* Fix gutters example styling

* Fix text colors on background utils docs

* redesign and fix up position marker example, which doesn't show nicely in darkmode but at least isn't broken

* fix some color utils examples

* Deprecate mixin notice

* Deprecate notice for list-group-item-variant() mixin

* Revamp new link CSS vars

* Use map-keys in some each Sass files

* Remove list-group-item-variant mixin ref in sass loop desc

* Display CSS vars scoped to our built-in dark mode

* Revert previous commit

* Fix list group variant link

* Fix typo

* Remove imports of alert/list-group mixins in scss/_mixins.scss

* Small formatting + comments removal in scss/_content.scss

* Fix alert links colors

* fix dropdown border-radius mixin

* fix link color and underline again, this time using CSS var override for color var and fallback value for the underline

* fix colors on docs navbar for dark mode

* remove two changes

* missing ref

* another link underline fix, just use sass vars for link decoration for now

* missing color bg docs, plus move dropdown override to scss

* more changes from review

* fix some examples, drop unused docs navbar styles, update docs navbar color mode to use mixin

* Few fixes around type

- Restored CSS variable for color on headings, this time with a fallback value
- In conjunction, restored and wrapped the default CSS var with a null value check
- Split headings and paragraphs docs in Reboot, elaborated on them

* Restyle custom details > summary element in docs

* Rewrite some migration docs

* fix form checks

* Fix up some navbar styling, tweak docs callout

* Fix select images, mostly for validation styling

* Clean up some migration notes, document some new form control CSS vars, mention new variables-dark in sass docs

* Update site/content/docs/5.2/components/scrollspy.md

Co-authored-by: Julien Déramond <julien.deramond@orange.com>

* Apply suggestions from code review

Co-authored-by: Julien Déramond <julien.deramond@orange.com>

* mention form control css vars in migration guide

* Tweak grid and flex docs background examples

* clarify some docs

* fix some more things

Co-authored-by: Julien Déramond <juderamond@gmail.com>
Co-authored-by: Julien Déramond <julien.deramond@orange.com>

site/content/docs/5.2/components/toasts.md

@@ -27,7 +27,7 @@ To encourage extensible and predictable toasts, we recommend a header and body.
 
 Toasts are as flexible as you need and have very little required markup. At a minimum, we require a single element to contain your "toasted" content and strongly encourage a dismiss button.
 
-{{< example class="bg-light" >}}
+{{< example >}}
 <div class="toast" role="alert" aria-live="assertive" aria-atomic="true">
   <div class="toast-header">
     {{< placeholder width="20" height="20" background="#007aff" class="rounded me-2" text="false" title="false" >}}
@@ -121,7 +121,7 @@ Toasts are slightly translucent to blend in with what's below them.
 
 You can stack toasts by wrapping them in a toast container, which will vertically add some spacing.
 
-{{< example class="bg-light" >}}
+{{< example >}}
 <div class="toast-container position-static">
   <div class="toast" role="alert" aria-live="assertive" aria-atomic="true">
     <div class="toast-header">
@@ -153,7 +153,7 @@ You can stack toasts by wrapping them in a toast container, which will verticall
 
 Customize your toasts by removing sub-components, tweaking them with [utilities]({{< docsref "/utilities/api" >}}), or by adding your own markup. Here we've created a simpler toast by removing the default `.toast-header`, adding a custom hide icon from [Bootstrap Icons]({{< param icons >}}), and using some [flexbox utilities]({{< docsref "/utilities/flex" >}}) to adjust the layout.
 
-{{< example class="bg-light" >}}
+{{< example >}}
 <div class="toast align-items-center" role="alert" aria-live="assertive" aria-atomic="true">
   <div class="d-flex">
     <div class="toast-body">
@@ -166,7 +166,7 @@ Customize your toasts by removing sub-components, tweaking them with [utilities]
 
 Alternatively, you can also add additional controls and components to toasts.
 
-{{< example class="bg-light" >}}
+{{< example >}}
 <div class="toast" role="alert" aria-live="assertive" aria-atomic="true">
   <div class="toast-body">
     Hello, world! This is a toast message.
@@ -182,7 +182,7 @@ Alternatively, you can also add additional controls and components to toasts.
 
 Building on the above example, you can create different toast color schemes with our [color]({{< docsref "/utilities/colors" >}}) and [background]({{< docsref "/utilities/background" >}}) utilities. Here we've added `.text-bg-primary` to the `.toast`, and then added `.btn-close-white` to our close button. For a crisp edge, we remove the default border with `.border-0`.
 
-{{< example class="bg-light" >}}
+{{< example >}}
 <div class="toast align-items-center text-bg-primary border-0" role="alert" aria-live="assertive" aria-atomic="true">
   <div class="d-flex">
     <div class="toast-body">
@@ -215,7 +215,7 @@ Place toasts with custom CSS as you need them. The top right is often used for n
     </select>
   </div>
 </form>
-<div aria-live="polite" aria-atomic="true" class="bg-dark position-relative bd-example-toasts">
+<div aria-live="polite" aria-atomic="true" class="bg-body-secondary position-relative bd-example-toasts rounded-3">
   <div class="toast-container p-3" id="toastPlacement">
     <div class="toast">
       <div class="toast-header">
@@ -233,7 +233,7 @@ Place toasts with custom CSS as you need them. The top right is often used for n
 
 For systems that generate more notifications, consider using a wrapping element so they can easily stack.
 
-{{< example class="bg-dark bd-example-toasts p-0" >}}
+{{< example class="bd-example-toasts p-0" >}}
 <div aria-live="polite" aria-atomic="true" class="position-relative">
   <!-- Position it: -->
   <!-- - `.toast-container` for spacing between toasts -->
@@ -271,7 +271,7 @@ For systems that generate more notifications, consider using a wrapping element
 
 You can also get fancy with flexbox utilities to align toasts horizontally and/or vertically.
 
-{{< example class="bg-dark bd-example-toasts d-flex" >}}
+{{< example class="bd-example-toasts d-flex" >}}
 <!-- Flexbox container for aligning the toasts -->
 <div aria-live="polite" aria-atomic="true" class="d-flex justify-content-center align-items-center w-100">
 
@@ -308,7 +308,7 @@ As the content you're displaying changes, be sure to update the [`delay` timeout
 
 When using `autohide: false`, you must add a close button to allow users to dismiss the toast.
 
-{{< example class="bg-light" >}}
+{{< example >}}
 <div role="alert" aria-live="assertive" aria-atomic="true" class="toast" data-bs-autohide="false">
   <div class="toast-header">
     {{< placeholder width="20" height="20" background="#007aff" class="rounded me-2" text="false" title="false" >}}