first draft

app/Resources/assets/scss/bootstrap/mixins/_progress-bar.scss

@@ -0,0 +1,10 @@
+// Progress bars
+
+@mixin progress-bar-variant($color) {
+  background-color: $color;
+
+  // Deprecated parent class requirement as of v3.2.0
+  .progress-striped & {
+    @include gradient-striped;
+  }
+}