Gradients and shadows (#24429)

* 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

1 files changed, 6 insertions, 0 deletions

diff --git a/scss/_carousel.scss b/scss/_carousel.scss
index 8aa14ace7..c3c207387 100644
--- a/scss/_carousel.scss
+++ b/scss/_carousel.scss
@@ -90,9 +90,15 @@
 }
 .carousel-control-prev {
   left: 0;
+  @if $enable-gradients {
+    background: linear-gradient(90deg, rgba(0,0,0,.25), rgba(0,0,0,.001));
+  }
 }
 .carousel-control-next {
   right: 0;
+  @if $enable-gradients {
+    background: linear-gradient(270deg, rgba(0,0,0,.25), rgba(0,0,0,.001));
+  }
 }
 
 // Icons for within