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/layouts/partials/footer.html

@@ -1,12 +1,12 @@
-<footer class="bd-footer py-4 py-md-5 mt-5 bg-light">
-  <div class="container py-4 py-md-5 px-4 px-md-3">
+<footer class="bd-footer py-4 py-md-5 mt-5 bg-body-tertiary">
+  <div class="container py-4 py-md-5 px-4 px-md-3 text-body-secondary">
     <div class="row">
       <div class="col-lg-3 mb-3">
-        <a class="d-inline-flex align-items-center mb-2 link-dark text-decoration-none" href="/" aria-label="Bootstrap">
+        <a class="d-inline-flex align-items-center mb-2 text-body-secondary text-decoration-none" href="/" aria-label="Bootstrap">
           {{ partial "icons/bootstrap-white-fill.svg" (dict "class" "d-block me-2" "width" "40" "height" "32") }}
           <span class="fs-5">Bootstrap</span>
         </a>
-        <ul class="list-unstyled small text-muted">
+        <ul class="list-unstyled small">
           <li class="mb-2">Designed and built with all the love in the world by the <a href="/docs/{{ .Site.Params.docs_version }}/about/team/">Bootstrap team</a> with the help of <a href="{{ .Site.Params.repo }}/graphs/contributors">our contributors</a>.</li>
           <li class="mb-2">Code licensed <a href="{{ .Site.Params.repo }}/blob/main/LICENSE" target="_blank" rel="license noopener">MIT</a>, docs <a href="https://creativecommons.org/licenses/by/3.0/" target="_blank" rel="license noopener">CC BY 3.0</a>.</li>
           <li class="mb-2">Currently v{{ .Site.Params.current_version }}.</li>