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/helpers/stacks.md

@@ -19,9 +19,9 @@ Use `.vstack` to create vertical layouts. Stacked items are full-width by defaul
 
 {{< example >}}
 <div class="vstack gap-3">
-  <div class="bg-light border">First item</div>
-  <div class="bg-light border">Second item</div>
-  <div class="bg-light border">Third item</div>
+  <div class="bg-body-tertiary border">First item</div>
+  <div class="bg-body-tertiary border">Second item</div>
+  <div class="bg-body-tertiary border">Third item</div>
 </div>
 {{< /example >}}
 
@@ -31,9 +31,9 @@ Use `.hstack` for horizontal layouts. Stacked items are vertically centered by d
 
 {{< example >}}
 <div class="hstack gap-3">
-  <div class="bg-light border">First item</div>
-  <div class="bg-light border">Second item</div>
-  <div class="bg-light border">Third item</div>
+  <div class="bg-body-tertiary border">First item</div>
+  <div class="bg-body-tertiary border">Second item</div>
+  <div class="bg-body-tertiary border">Third item</div>
 </div>
 {{< /example >}}
 
@@ -41,9 +41,9 @@ Using horizontal margin utilities like `.ms-auto` as spacers:
 
 {{< example >}}
 <div class="hstack gap-3">
-  <div class="bg-light border">First item</div>
-  <div class="bg-light border ms-auto">Second item</div>
-  <div class="bg-light border">Third item</div>
+  <div class="bg-body-tertiary border">First item</div>
+  <div class="bg-body-tertiary border ms-auto">Second item</div>
+  <div class="bg-body-tertiary border">Third item</div>
 </div>
 {{< /example >}}
 
@@ -51,10 +51,10 @@ And with [vertical rules]({{< docsref "/helpers/vertical-rule" >}}):
 
 {{< example >}}
 <div class="hstack gap-3">
-  <div class="bg-light border">First item</div>
-  <div class="bg-light border ms-auto">Second item</div>
+  <div class="bg-body-tertiary border">First item</div>
+  <div class="bg-body-tertiary border ms-auto">Second item</div>
   <div class="vr"></div>
-  <div class="bg-light border">Third item</div>
+  <div class="bg-body-tertiary border">Third item</div>
 </div>
 {{< /example >}}
 

site/content/docs/5.2/helpers/stretched-link.md

@@ -36,7 +36,7 @@ Most custom components do not have `position: relative` by default, so we need t
 {{< /example >}}
 
 {{< example >}}
-<div class="row g-0 bg-light position-relative">
+<div class="row g-0 bg-body-secondary position-relative">
   <div class="col-md-6 mb-md-0 p-md-4">
     {{< placeholder width="100%" height="200" class="w-100" text="false" title="Generic placeholder image" >}}
   </div>

site/content/docs/5.2/helpers/vertical-rule.md

@@ -37,9 +37,9 @@ They can also be used in [stacks]({{< docsref "/helpers/stacks" >}}):
 
 {{< example >}}
 <div class="hstack gap-3">
-  <div class="bg-light border">First item</div>
-  <div class="bg-light border ms-auto">Second item</div>
+  <div class="bg-body-tertiary border">First item</div>
+  <div class="bg-body-tertiary border ms-auto">Second item</div>
   <div class="vr"></div>
-  <div class="bg-light border">Third item</div>
+  <div class="bg-body-tertiary border">Third item</div>
 </div>
 {{< /example >}}