[enh] static content generalization

searx/static/less/bootstrap/mixins/progress-bar.less

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