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

The option is set to `always-single-line` in stylelint-config-standard which we extend.

We don't change anything so the transition is basically unused.

* 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 and hover state

This way, descenders are not cropped when truncating. Fixes #23956.

* font-sizes relative to font-size-base

* ditch $heading-font-size-base, simplify

* apply same to .lead var

* also update progress bar font-size var

Using `rgba($white,1)` results in `white` instead of the hex.

Now it's more consistent.

Also switch to numeric values for font-weight.

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

(#22414) Rename variables `$table-inverse-bg`, `$table-inverse-accent-bg`, `$table-inverse-hover-bg`, `$table-inverse-border-color`, `$table-inverse-color` to `$table-dark-bg`, `$table-dark-accent-bg`, `$table-dark-hover-bg`, `$table-dark-border-color`, `$table-dark-color` for consistency

I never use this dang list and it just falls out of date far too easily. Dropping for my own sanity.

There is no _custom.scss file anymore.