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/_navbar.scss

@@ -1,8 +1,16 @@
 .bd-navbar {
   padding: .75rem 0;
   background-color: transparent;
-  background-image: linear-gradient(to bottom, rgba(var(--bd-violet-rgb), 1), rgba(var(--bd-violet-rgb), .95));
-  box-shadow: 0 .5rem 1rem rgba(0, 0, 0, .15), inset 0 -1px 0 rgba(0, 0, 0, .15);
+  box-shadow: 0 .5rem 1rem rgba($black, .15), inset 0 -1px 0 rgba($white, .15);
+
+  &::after {
+    position: absolute;
+    inset: 0;
+    z-index: -1;
+    display: block;
+    content: "";
+    background-image: linear-gradient(to bottom, rgba(var(--bd-violet-rgb), 1), rgba(var(--bd-violet-rgb), .95));
+  }
 
   .bd-navbar-toggle {
     @include media-breakpoint-down(lg) {
@@ -30,6 +38,7 @@
   }
 
   .navbar-brand {
+    color: $white;
     transition: .2s ease-in-out transform; // stylelint-disable-line property-disallowed-list
 
     &:hover {
@@ -60,7 +69,7 @@
   }
 
   .offcanvas-lg {
-    background-color: var(--bd-violet);
+    background-color: var(--bd-violet-bg);
     border-left: 0;
 
     @include media-breakpoint-down(lg) {
@@ -75,17 +84,42 @@
   }
 
   .dropdown-menu {
-    --#{$prefix}dropdown-min-width: 12rem;
-    --#{$prefix}dropdown-link-hover-bg: rgba(var(--bd-violet-rgb), .1);
-    @include rfs(.875rem, --#{$prefix}dropdown-font-size);
+    --bs-dropdown-min-width: 12rem;
+    --bs-dropdown-padding-x: .25rem;
+    --bs-dropdown-padding-y: .25rem;
+    --bs-dropdown-link-hover-bg: rgba(var(--bd-violet-rgb), .1);
+    --bs-dropdown-link-active-bg: rgba(var(--bd-violet-rgb), 1);
+    @include rfs(.875rem, --bs-dropdown-font-size);
+    @include font-size(.875rem);
+    @include border-radius(.5rem);
     box-shadow: $dropdown-box-shadow;
-  }
 
-  .dropdown-item.current {
-    font-weight: 600;
-    background-image: escape-svg($dropdown-active-icon);
-    background-repeat: no-repeat;
-    background-position: right $dropdown-item-padding-x top .6rem;
-    background-size: .75rem .75rem;
+    li + li {
+      margin-top: .125rem;
+    }
+
+    .dropdown-item {
+      @include border-radius(.25rem);
+
+      &:active {
+        .bi {
+          color: inherit !important; // stylelint-disable-line declaration-no-important
+        }
+      }
+    }
+
+    .active {
+      font-weight: 600;
+
+      .bi {
+        display: block !important; // stylelint-disable-line declaration-no-important
+      }
+    }
+  }
+}
+
+@include color-mode(dark) {
+  .bd-navbar {
+    box-shadow: 0 .5rem 1rem rgba($black, .15), inset 0 -1px 0 rgba($white, .15);
   }
 }