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/_content.scss

@@ -32,6 +32,9 @@
   // Override Bootstrap defaults
   > .table,
   > .table-responsive .table {
+    --bs-table-border-color: var(--bs-border-color);
+
+    max-width: 100%;
     margin-bottom: 1.5rem;
     @include font-size(.875rem);
 
@@ -60,6 +63,14 @@
       }
     }
 
+    th {
+      color: var(--bs-emphasis-color);
+    }
+
+    strong {
+      color: var(--bs-emphasis-color);
+    }
+
     // Prevent breaking of code
     // stylelint-disable-next-line selector-max-compound-selectors
     th,
@@ -80,6 +91,16 @@
   min-width: 280px;
 }
 
+.table-swatches {
+  th {
+    color: var(--bs-emphasis-color);
+  }
+
+  td code {
+    white-space: nowrap;
+  }
+}
+
 .bd-title {
   @include font-size(3rem);
 }
@@ -89,13 +110,10 @@
   font-weight: 300;
 }
 
-.bd-bg-violet {
-  background-color: $bd-violet;
-}
-
 .bi {
   width: 1em;
   height: 1em;
+  vertical-align: -.125em;
   fill: currentcolor;
 }
 
@@ -121,6 +139,43 @@
 
 .border-lg-start {
   @include media-breakpoint-up(lg) {
-    border-left: $border-width solid $border-color;
+    border-left: var(--bs-border-width) solid var(--bs-border-color);
   }
 }
+
+// stylelint-disable selector-no-qualifying-type
+.bd-summary-link {
+  color: var(--bs-link-color);
+
+  &:hover,
+  details[open] > & {
+    color: var(--bs-link-hover-color);
+  }
+}
+// stylelint-enable selector-no-qualifying-type
+
+// scss-docs-start custom-color-mode
+[data-bs-theme="blue"] {
+  --bs-body-color: var(--bs-white);
+  --bs-body-color-rgb: #{to-rgb($white)};
+  --bs-body-bg: var(--bs-blue);
+  --bs-body-bg-rgb: #{to-rgb($blue)};
+  --bs-tertiary-bg: #{$blue-600};
+
+  .dropdown-menu {
+    --bs-dropdown-bg: #{mix($blue-500, $blue-600)};
+    --bs-dropdown-link-active-bg: #{$blue-700};
+  }
+
+  .btn-secondary {
+    --bs-btn-bg: #{mix($gray-600, $blue-400, .5)};
+    --bs-btn-border-color: #{rgba($white, .25)};
+    --bs-btn-hover-bg: #{darken(mix($gray-600, $blue-400, .5), 5%)};
+    --bs-btn-hover-border-color: #{rgba($white, .25)};
+    --bs-btn-active-bg: #{darken(mix($gray-600, $blue-400, .5), 10%)};
+    --bs-btn-active-border-color: #{rgba($white, .5)};
+    --bs-btn-focus-border-color: #{rgba($white, .5)};
+    --bs-btn-focus-box-shadow: 0 0 0 .25rem rgba(255, 255, 255, .2);
+  }
+}
+// scss-docs-end custom-color-mode