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/assets/scss/_sidebar.scss

@@ -11,6 +11,17 @@
     margin-left: -.25rem;
     overflow-y: auto;
   }
+
+  @include media-breakpoint-down(lg) {
+    .offcanvas-lg {
+      border-right-color: var(--bs-border-color);
+      box-shadow: $box-shadow-lg;
+    }
+  }
+}
+
+.bd-links-heading {
+  color: var(--bs-emphasis-color);
 }
 
 .bd-links-nav {
@@ -35,16 +46,16 @@
 .bd-links-link {
   padding: .1875rem .5rem;
   margin-top: .125rem;
-  margin-left: 1rem;
-  color: rgba($black, .65);
+  margin-left: 1.125rem;
+  color: var(--bs-body-color);
   text-decoration: if($link-decoration == none, null, none);
 
   &:hover,
   &:focus,
   &.active {
-    color: rgba($black, .85);
+    color: var(--bs-emphasis-color);
     text-decoration: if($link-hover-decoration == underline, none, null);
-    background-color: rgba(var(--bd-violet-rgb), .1);
+    background-color: var(--bd-sidebar-link-bg);
   }
 
   &.active {