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/about/brand.md

@@ -12,13 +12,13 @@ Have a need for Bootstrap's brand resources? Great! We have only a few guideline
 
 When referencing Bootstrap, use our logo mark. Do not modify our logos in any way. Do not use Bootstrap's branding for your own open or closed source projects. **Do not use the Twitter name or logo** in association with Bootstrap.
 
-<div class="bd-brand-item px-2 py-5 mb-3 bg-light rounded-lg">
+<div class="bd-brand-item px-2 py-5 mb-3 border rounded-3">
   <img class="d-block img-fluid mx-auto" src="/docs/{{< param docs_version >}}/assets/brand/bootstrap-logo.svg" alt="Bootstrap" width="256" height="204">
 </div>
 
 Our logo mark is also available in black and white. All rules for our primary logo apply to these as well.
 
-<div class="bd-brand-logos d-sm-flex text-center bg-light rounded-lg overflow-hidden w-100 mb-3">
+<div class="bd-brand-logos d-sm-flex text-center bg-light rounded-3 overflow-hidden w-100 mb-3">
   <div class="bd-brand-item w-100 px-2 py-5">
     <img src="/docs/{{< param docs_version >}}/assets/brand/bootstrap-logo-black.svg" alt="Bootstrap" width="128" height="102" loading="lazy">
   </div>
@@ -31,7 +31,7 @@ Our logo mark is also available in black and white. All rules for our primary lo
 
 Bootstrap should always be referred to as just **Bootstrap**. No Twitter before it and no capital _s_.
 
-<div class="bd-brand-logos d-sm-flex text-center bg-light rounded-lg overflow-hidden w-100 mb-3">
+<div class="bd-brand-logos d-sm-flex text-center border rounded-3 overflow-hidden w-100 mb-3">
   <div class="bd-brand-item w-100 p-3">
     <div class="h3">Bootstrap</div>
     <strong class="text-success">Correct</strong>