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

scss/mixins/_alert.scss

@@ -1,6 +1,6 @@
 @mixin alert-variant($background, $border, $color) {
   color: $color;
-  background-color: $background;
+  @include gradient-bg($background);
   border-color: $border;
 
   hr {

scss/mixins/_background-variant.scss

@@ -12,3 +12,9 @@
     }
   }
 }
+
+@mixin bg-gradient-variant($parent, $color) {
+  #{$parent} {
+    background: $color linear-gradient(180deg, mix($body-bg, $color, 15%), $color) repeat-x !important;
+  }
+}

scss/mixins/_buttons.scss

@@ -5,13 +5,13 @@
 
 @mixin button-variant($background, $border, $hover-background: darken($background, 7.5%), $hover-border: darken($border, 10%), $active-background: darken($background, 10%), $active-border: darken($border, 12.5%)) {
   color: color-yiq($background);
-  background-color: $background;
+  @include gradient-bg($background);
   border-color: $border;
   @include box-shadow($btn-box-shadow);
 
   @include hover {
     color: color-yiq($hover-background);
-    background-color: $hover-background;
+    @include gradient-bg($hover-background);
     border-color: $hover-border;
   }
 
@@ -19,9 +19,9 @@
   &.focus {
     // Avoid using mixin so we can pass custom focus shadow properly
     @if $enable-shadows {
-      box-shadow: $btn-box-shadow, 0 0 0 3px rgba($border, .5);
+      box-shadow: $btn-box-shadow, 0 0 0 $input-btn-focus-width rgba($border, .5);
     } @else {
-      box-shadow: 0 0 0 3px rgba($border, .5);
+      box-shadow: 0 0 0 $input-btn-focus-width rgba($border, .5);
     }
   }
 
@@ -35,10 +35,19 @@
   &:not([disabled]):not(.disabled):active,
   &:not([disabled]):not(.disabled).active,
   .show > &.dropdown-toggle {
+    color: color-yiq($active-background);
     background-color: $active-background;
-    background-image: none; // Remove the gradient for the pressed/active state
+    @if $enable-gradients {
+      background-image: none; // Remove the gradient for the pressed/active state
+    }
     border-color: $active-border;
-    @include box-shadow($btn-active-box-shadow);
+
+    // Avoid using mixin so we can pass custom focus shadow properly
+    @if $enable-shadows {
+      box-shadow: $btn-active-box-shadow, 0 0 0 $input-btn-focus-width rgba($border, .5);
+    } @else {
+      box-shadow: 0 0 0 $input-btn-focus-width rgba($border, .5);
+    }
   }
 }
 
@@ -56,7 +65,7 @@
 
   &:focus,
   &.focus {
-    box-shadow: 0 0 0 3px rgba($color, .5);
+    box-shadow: 0 0 0 $input-btn-focus-width rgba($color, .5);
   }
 
   &.disabled,
@@ -71,6 +80,8 @@
     color: $color-hover;
     background-color: $color;
     border-color: $color;
+    // Avoid using mixin so we can pass custom focus shadow properly
+    box-shadow: 0 0 0 $input-btn-focus-width rgba($color, .5);
   }
 }
 

scss/mixins/_forms.scss

@@ -16,7 +16,12 @@
     background-color: $input-focus-bg;
     border-color: $input-focus-border-color;
     outline: none;
-    @include box-shadow($input-focus-box-shadow);
+    // Avoid using mixin so we can pass custom focus shadow properly
+    @if $enable-shadows {
+      box-shadow: $input-box-shadow, $input-btn-focus-box-shadow;
+    } @else {
+      box-shadow: $input-btn-focus-box-shadow;
+    }
   }
 }
 

scss/mixins/_gradients.scss

@@ -1,5 +1,13 @@
 // Gradients
 
+@mixin gradient-bg($color) {
+  @if $enable-gradients {
+    background: $color linear-gradient(180deg, mix($body-bg, $color, 15%), $color) repeat-x;
+  } @else {
+    background-color: $color;
+  }
+}
+
 // Horizontal gradient, from left to right
 //
 // Creates two color stops, start and end, by specifying a color and position for each color stop.