Bootstrap5 (#17)

* Bootstrap5 migration

dev/scss/bootstrap/_close.scss

@@ -1,40 +1,40 @@
-.close {
-  float: right;
-  @include font-size($close-font-size);
-  font-weight: $close-font-weight;
-  line-height: 1;
-  color: $close-color;
-  text-shadow: $close-text-shadow;
-  opacity: .5;
-
-  // Override <a>'s hover style
-  @include hover() {
-    color: $close-color;
-    text-decoration: none;
-  }
-
-  &:not(:disabled):not(.disabled) {
-    @include hover-focus() {
-      opacity: .75;
-    }
-  }
-}
-
-// Additional properties for button version
+// transparent background and border properties included for button version.
 // iOS requires the button element instead of an anchor tag.
 // If you want the anchor version, it requires `href="#"`.
 // See https://developer.mozilla.org/en-US/docs/Web/Events/click#Safari_Mobile
 
-// stylelint-disable-next-line selector-no-qualifying-type
-button.close {
-  padding: 0;
-  background-color: transparent;
-  border: 0;
+.btn-close {
+  box-sizing: content-box;
+  width: $btn-close-width;
+  height: $btn-close-height;
+  padding: $btn-close-padding-y $btn-close-padding-x;
+  color: $btn-close-color;
+  background: transparent escape-svg($btn-close-bg) center / $btn-close-width auto no-repeat; // include transparent for button elements
+  border: 0; // for button elements
+  @include border-radius();
+  opacity: $btn-close-opacity;
+
+  // Override <a>'s hover style
+  &:hover {
+    color: $btn-close-color;
+    text-decoration: none;
+    opacity: $btn-close-hover-opacity;
+  }
+
+  &:focus {
+    outline: 0;
+    box-shadow: $btn-close-focus-shadow;
+    opacity: $btn-close-focus-opacity;
+  }
+
+  &:disabled,
+  &.disabled {
+    pointer-events: none;
+    user-select: none;
+    opacity: $btn-close-disabled-opacity;
+  }
 }
 
-// Future-proof disabling of clicks on `<a>` elements
-
-// stylelint-disable-next-line selector-no-qualifying-type
-a.close.disabled {
-  pointer-events: none;
+.btn-close-white {
+  filter: $btn-close-white-filter;
 }