Minor edits for style and consistency

Fixes #25669, closes #25779.

Running `./build/change-version.js v4.0.0-alpha.6 v4.0.0` fixed this,
so the version change script works fine. I'm presuming instead this
change was just omitted from 35f80bb1, and then wouldn't have
been caught by subsequent runs of `change-version`, since it only
ever replaces the exact old version string specified.

See issue #25711

* load skippy markup from includes

This variable is not used anywhere.

Leftover from #18373.

As discussed in issue's comment:
https://git.io/vAuPW

Thanks!

This file no longer has intentional indentation errors

This adds `form-group` to add the same vertical space to the `form-check` as in other grouped controls
used in form sample markup.

Thanks!

* Tell where to find the variables to override

* Be consistent with path to _variables.scss

* Get rid of some 'our' in the doc

Replaced by 'Boostrap's' when relevant

* 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

For consistency. Closes https://github.com/twbs/bootstrap/issues/25642

[Docs] Clarify suggested use of custom validation styles, add note about resetting form appearance after validation (#25628)

* Expand/clarify recommendation for using custom validation styles

Seems support for correctly announcing browser default navigation bubbles/notifications has improved recently. Now it seems only Chrome (at least on Win and Android, not tested on macOS) is the odd one out.

However, we should really make sure the custom validation messages are also tied correctly to their relevant form controls - need to dig out the PR that will ensure that.

* Add note about resetting form appearance after submission

xref https://github.com/twbs/bootstrap/issues/25626

Closes https://github.com/twbs/bootstrap/issues/25611

2 small typos

Fixes #25235

* Dropdown - Allow to disable Popper.js style

* Update dropdown.js

* Update dropdown.html

* copy changes

* add missing properties

list of new properties:
align-content
animation-fill-mode
appearance
column-count
column-fill
column-gap
column-rule
column-rule-color
column-rule-style
column-rule-width
column-span
column-width
columns
direction
fill
flex-shrink
orphans
page-break-after
page-break-before
page-break-inside
perspective
src
stroke
unicode-bidi
widows

* fix invalid JSON config

add curly brackets

* Update .stylelintrc

* Remove clip-path from .sr-only utility as it causes perf regressions in Chrome

* change snippet to example