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>

scss/_utilities.scss

@@ -163,6 +163,11 @@ $utilities: map-merge(
       ),
       values: $utilities-border-colors
     ),
+    "subtle-border-color": (
+      property: border-color,
+      class: border,
+      values: $utilities-border-subtle
+    ),
     "border-width": (
       css-var: true,
       css-variable-name: border-width,
@@ -570,9 +575,12 @@ $utilities: map-merge(
       values: map-merge(
         $utilities-text-colors,
         (
-          "muted": $text-muted,
+          "muted": $text-muted, // deprecated
           "black-50": rgba($black, .5), // deprecated
           "white-50": rgba($white, .5), // deprecated
+          "body-secondary": var(--#{$prefix}secondary-color),
+          "body-tertiary": var(--#{$prefix}tertiary-color),
+          "body-emphasis": var(--#{$prefix}emphasis-color),
           "reset": inherit,
         )
       )
@@ -587,6 +595,11 @@ $utilities: map-merge(
         100: 1
       )
     ),
+    "text-color": (
+      property: color,
+      class: text,
+      values: $utilities-text-emphasis-colors
+    ),
     // scss-docs-end utils-color
     // scss-docs-start utils-bg-color
     "background-color": (
@@ -598,7 +611,10 @@ $utilities: map-merge(
       values: map-merge(
         $utilities-bg-colors,
         (
-          "transparent": transparent
+          "transparent": transparent,
+          "body-secondary": rgba(var(--#{$prefix}secondary-bg-rgb), var(--#{$prefix}bg-opacity)),
+          "body-tertiary": rgba(var(--#{$prefix}tertiary-bg-rgb), var(--#{$prefix}bg-opacity)),
+          "body-emphasis": rgba(var(--#{$prefix}emphasis-bg-rgb), var(--#{$prefix}bg-opacity)),
         )
       )
     ),
@@ -613,6 +629,11 @@ $utilities: map-merge(
         100: 1
       )
     ),
+    "subtle-background-color": (
+      property: background-color,
+      class: bg,
+      values: $utilities-bg-subtle
+    ),
     // scss-docs-end utils-bg-color
     "gradient": (
       property: background-image,