Drop `color()`, `theme-color()` & `gray()` functions in favor of variables. The functions just called a `map-get()` of a map where just the variables were defined.
Also the `theme-color-level()` now accepts any color you want instead of only `$theme-colors` colors. The first value now is a variable instead of the `$theme-colors` key.
Media queries do not support CSS custom properties (CSS variables), so we've had some useless variables lying around since shipping v4. This finally removes them and the associated documentation content that explained the situation.
Fixes#29012.
* Initial spike of consolidated form checks
* Stub out forms rearrangement
- Prepping to drop non-custom file and range inputs
- Prepping to merge custom and native checks and radios (with switches)
- Prepping to merge custom select with form select
- Moving docs arround so forms has it's own area given volume of CSS
* Move input group Sass file to forms subdir
* Start to split and move the docs around
* Simpler imports
* Copyediting
* delete overview file
* Remove commented out code
* remove the custom-forms import
* rewrite flex-check as form-check, replace all custom properties
* Remove old forms doc
* stub out new subpage link section
* update migration guide
* Update nav, forms overview in page nav, and descriptions
* fix check bg position
* fix margin-top calculation
* rename .custom-select to .form-select
* Update validation styles for new checks
* add some vertical margin, fix inline checks
* fix docs examples
* better way to do this contents stuff, redo the toc while i'm at it
* page restyle for docs while here
* un-callout that, edit text
* redo padding on toc
* fix toc
* start to cleanup checks docs
* Rewrite Markdown tables into HTML
* Redesign tables, redo their docs
* Replace Open Iconic icons with custom Bootstrap icons
* Redesign the docs navbar, add a subheader, redo the sidebar
* Redesign docs homepage a bit
* Simplify table style overrides for docs tables
* Simplify docs typography for page titles and reading line length
* Stub out icons page
* Part of sidebar update, remove migration from nav.yml
* Move toc CSS to separate partial
* Change appearance of overview page
* fix sidebar arrow direction
* Add footer to docs layout
* Update descriptions
* Drop the .form-group class for margin utilities
* Remove lingering form-group-margin-bottom var
* improve footer spacing
* add headings to range page
* uncomment form range css
* Rename .custom-range to .form-range
* Drop unused docs var
* Uncomment the comment
* Remove unused variable
* Fix radio image sizing
* Reboot update: reset horizontal ul and ol padding
* de-dupe IDs
* tweak toc styles
* nvm, fix dropdown versions stuff
* remove sidebar nav toggle for now
* broken html
* fix more broken html, move css
* scss linting
* comment out broken helper docs
* scope styles
* scope styles
* Fixes#25540 and fixes#26407 for v5 only
* Update sidebar once more
* Match new sidenav order
* fix syntax error
* Rename custom-file to form-file, update paths, update migration docs for previous changes in #28696
* rename back
* fix size and alignment
* rename that back too
