Fixes #26897

* Fix focus box shadow of custom ranges

* Put `:focus` selector right after `.custom-range`
* Remove unnecessary `outline: none`

* Fix box-shadow issues in IE/Edge

* Better align

* Fix thumb vertical positions in IE/Edge

* Fix incorrect formula of thumb vertical positions in Webkit

* Fix an incorrect comment

* Fix incorrect box-shadow-width

Now `.custom-control` will have height equal to `$font-size-base`
Fix custom checkbox and radio top position (we can't subtract rem from unit)
`$line-height-base - $custom-control-indicator-size` is not valid
Because `$line-height-base` is a unitless variable and `$custom-control-indicator-size`
is a unit (rem) variable

- Fixes #25656 where custom selects had the wrong focus shadow
- Fixes #26668 where custom selects had no inset shadow (also closes #26572 which had the wrong form classes on the first example anyway)

* added the styling

* added the documentation

* update for one rule per line

* fix hound error: trailing whitespace

* trimmed off vendor prefixes

* Add note about track and thumb

* Psuedo-elements must be split across multiple rulesets to have an affect

* Fix firefox inner focus

* Seems that FF is the only one affected by this

* Add support for gradients

* Add labels, clarify min/max changes

* add step example

* add custom range vars

added variable for the background of checked and indeterminate custom inputs and radios that are disabled
added styles for these cases

- Changes the wrapping label to a div so we can style the label instead of another element while also supporting form validation.
- Fixes form validation styles for custom file input (closes #24831).
- Updates docs with validation styles (also adding example feedback text while I was there) and new how it works section.

Matches browser default inputs and swaps some max-width properties for a regular width

* match layout behaviors

* ditch the indicator as separate element for psuedo-elements on the label

* move disabled to attribute only on input

* redo default inline check to support new markup

* redo inline forms

* clean up vars

* update validation mixin to new structure

* update checks in docs

* linting for for/id attributes

* Add support for size attribute on custom selects

* Add large custom select, document it and the small variant

* fix custom select focus state

* fix custom file input focus styles

* remove empty line

* Rewrite custom form check backgrounds

Previously, this was all just a background-color and background-image. When we restored the gradients though, we had two background-images competing on the same element, causing rendering glitches. This refactors that code, creating a mixin to simplify things, so we can we easily use two background-images (SVG icon and gradient) when -gradients is set to true.

Fixes #24598

* restore default vars

* Revamp custom check and radio backgrounds

Instead of applying multiple background-image's to the same element, we're adding a new ::before pseudo-element to layer the background-images. Gradients go on the .custom-control-indicator while their icons go on the ::before element. This allows us to shave some bytes from when we compile and we previously needed to redeclare the background-image for the icon if you changed the gradient.

* remove now unused mixin

* mention change in migration docs

* use `background-color` instead of the shorthand
* use `outline: 0` consistently
* fix transform order
* remove quotes from `SFMono-Regular`

* Update the form focus mixin to use a manual `$enable-shadows` check so we can always ensure a focus state for accessibility and consistency

* - Add new `$input-btn-focus-width` and `$input-btn-focus-color` variables.

- Replace separate `$btn-focus-box-shadow` and `$input-focus-box-shadow`
variables with unified `$input-btn-focus-box-shadow` to match our
combined variables approach elsewhere.

* Put new focus width var to use in buttons mixins

* use new button input-box shadow var

* Add a new mixin for quickly adding linear gradient to components when $enable-gradients is set to true

* use correct var

* fix focus shadows in button mixins

* Add opt-in gradients to alerts, buttons, carousel, custom radios and checkboxes, custom file input, and dropdown items

* Generate .bg-gradient- utilities

* add headings to colors page and document bg-gradient utils

* update the button color for active status, check with yiq as it's done for basic state an...

Turns out #22612 is a bad idea and it makes all sorts of weird visual
glitches when working with form controls and transparent borders.

Custom File `$custom-file-focus-box-shadow` should show even if box shadows are disabled

Custom File variables should inherit from inputs

c3e4cbdf changed the border of input elements to be translucent. This commit makes
input group elements and custom form inputs follow the same pattern.

(#22414) Rename for consistency `$custom-checkbox-border-radius`, `$custom-checkbox-icon-checked`, `$custom-checkbox-indeterminate-bg`, `$custom-checkbox-icon-indeterminate`, `$custom-checkbox-indeterminate-box-shadow`, `$custom-radio-border-radius`, `$custom-radio-icon-checked` to `$custom-checkbox-indicator-border-radius`, `$custom-checkbox-indicator-icon-checked`, `$custom-checkbox-indicator-indeterminate-bg`, `$custom-checkbox-indicator-icon-indeterminate`, `$custom-checkbox-indicator-indeterminate-box-shadow`, `$custom-radio-indicator-border-radius`, `$custom-radio-indicator-icon-checked`, respectively

(Fixes #22414) Rename for consistency `$custom-checkbox-radius`, `$custom-checkbox-checked-icon`, `$custom-checkbox-indeterminate-indicator-color`, `$custom-checkbox-indeterminate-icon`, `$custom-radio-radius`, `$custom-radio-checked-icon`, `$custom-select-sm-font-size`, to `$custom-checkbox-border-radius`, `$custom-checkbox-icon-checked`, `$custom-checkbox-indicator-indeterminate-color`, `$custom-checkbox-icon-indeterminate`, `$custom-radio-border-radius`, `$custom-radio-icon-checked`, `$custom-select-font-size-sm` respectively

(#22414) Rename for consistency `$custom-control-disabled-indicator-bg`, `$custom-control-disabled-description-color`, `$custom-control-checked-indicator-color`, `$custom-control-checked-indicator-bg`, `$custom-control-checked-indicator-box-shadow`, `$custom-control-focus-indicator-box-shadow`, `$custom-control-active-indicator-color`, `$custom-control-active-indicator-bg`, `$custom-control-active-indicator-box-shadow` to `$custom-control-indicator-disabled-bg`, `$custom-control-description-disabled-color`, `$custom-control-indicator-checked-color`, `$custom-control-indicator-checked-bg`, `$custom-control-indicator-checked-box-shadow`, `$custom-control-indicator-focus-box-shadow`, `$custom-control-indicator-active-color`, `$custom-control-indicator-active-bg`, `$custom-control-indicator-active-box-shadow`, respectively

— Sizing shorthand comes last
— Add height var and put it to use

— Previously we weren't including the border-width on the computed height, leading to alignment issues.

— New system utilizes three variables (not ideal, but straightforward) for computing these heights. One for the vertical border, one for the line-height/font-size/padding dance, and one to add those together.

— Updates CSS across forms and custom forms to use new sizing. Special note here: form validation icon sizing uses the inner variables because background-image doesn't bleed into borders unless explicit background-clip.

<label class="custom-file"><input type="file" class="custom-file-input" /><span class="custom-file-control"></span></label>
VS
<label class="custom-file"><input type="file" class="custom-file-input" /><span class="custom-file-control">test.txt</span></label>

* Fixed some linting issues

* Run npm tasks after scss cleanup

* Revert "Run npm tasks after scss cleanup"

This reverts commit 1103a0da.

* Property sort order for grid

* Let's respest the property order in the mixins

* Respect property sort order in reboot file

* ::-ms-expand is a vendor-prefix, add it to the scss-lint disable

* Revert hover mixin comment

* Fixed missing mixin hover-focus

In addition to #22011 and #21994