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/customize/sass.md

@@ -57,8 +57,9 @@ In your `custom.scss`, you'll import Bootstrap's source Sass files. You have two
 
 // 2. Include any default variable overrides here
 
-// 3. Include remainder of required Bootstrap stylesheets
+// 3. Include remainder of required Bootstrap stylesheets (including any separate color mode stylesheets)
 @import "../node_modules/bootstrap/scss/variables";
+@import "../node_modules/bootstrap/scss/variables-dark";
 
 // 4. Include any default map overrides here
 
@@ -104,6 +105,7 @@ $body-color: #111;
 
 // Required
 @import "../node_modules/bootstrap/scss/variables";
+@import "../node_modules/bootstrap/scss/variables-dark";
 @import "../node_modules/bootstrap/scss/maps";
 @import "../node_modules/bootstrap/scss/mixins";
 @import "../node_modules/bootstrap/scss/root";
@@ -166,6 +168,7 @@ To remove colors from `$theme-colors`, or any other map, use `map-remove`. Be aw
 // Required
 @import "../node_modules/bootstrap/scss/functions";
 @import "../node_modules/bootstrap/scss/variables";
+@import "../node_modules/bootstrap/scss/variables-dark";
 
 $theme-colors: map-remove($theme-colors, "info", "light", "dark");
 
@@ -294,7 +297,7 @@ Our `scss/mixins/` directory has a ton of mixins that power parts of Bootstrap a
 
 ### Color schemes
 
-A shorthand mixin for the `prefers-color-scheme` media query is available with support for `light`, `dark`, and custom color schemes.
+A shorthand mixin for the `prefers-color-scheme` media query is available with support for `light`, `dark`, and custom color schemes. See [the color modes documentation]({{< docsref "/customize/color-modes" >}}) for information on our color mode mixin.
 
 {{< scss-docs name="mixin-color-scheme" file="scss/mixins/_color-scheme.scss" >}}