Monorepo structure, remove Gulp, new build process (#2116)

core/scss/_bootstrap-override.scss

@@ -0,0 +1,78 @@
+@mixin caret($direction: down) {
+  $selector: "after";
+
+  @if $direction == "left" {
+    $selector: "before";
+  }
+
+  &:#{$selector} {
+    content: "";
+    display: inline-block;
+    vertical-align: $caret-vertical-align;
+    width: $caret-width;
+    height: $caret-width;
+    border-bottom: 1px var(--#{$prefix}border-style);
+    border-left: 1px var(--#{$prefix}border-style);
+    margin-right: 0.1em;
+
+    @if $direction != "left" {
+      margin-left: $caret-spacing;
+    } @else {
+      margin-right: $caret-spacing;
+    }
+
+    @if $direction == down {
+      transform: rotate(-45deg);
+    } @else if $direction == up {
+      transform: rotate(135deg);
+    } @else if $direction == end {
+      transform: rotate(-135deg);
+    } @else {
+      transform: rotate(45deg);
+    }
+  }
+
+  @if $direction == "left" {
+    &:after {
+      content: none;
+    }
+  }
+}
+
+@mixin alert-variant($background: null, $border: null, $color: null) {
+  // Override bootstrap core
+}
+
+@mixin button-variant(
+  $background: null,
+  $border: null,
+  $color: null,
+  $hover-background: null,
+  $hover-border: null,
+  $hover-color: null,
+  $active-background: null,
+  $active-border: null,
+  $active-color: null,
+  $disabled-background: null,
+  $disabled-border: null,
+  $disabled-color: null
+) {
+  // Override bootstrap core
+}
+
+@mixin button-outline-variant(
+  $color: null,
+  $color-hover: null,
+  $active-background: null,
+  $active-border: null,
+  $active-color: null
+) {
+  // Override bootstrap core
+}
+
+//
+// TODO: remove when https://github.com/twbs/bootstrap/pull/37425/ will be released
+//
+@function opaque($background, $foreground) {
+  @return mix(rgba($foreground, 1), $background, opacity($foreground) * 100%);
+}