Add background to MapboxVector layer

src/ol/render/Feature.js

@@ -42,6 +42,11 @@ class RenderFeature {
    * @param {number|string|undefined} id Feature id.
    */
   constructor(type, flatCoordinates, ends, properties, id) {
+    /**
+     * @type {import("../style/Style.js").StyleFunction|undefined}
+     */
+    this.styleFunction;
+
     /**
      * @private
      * @type {import("../extent.js").Extent|undefined}
@@ -259,10 +264,10 @@ class RenderFeature {
   }
 
   /**
-   * @return {undefined}
+   * @return {import('../style/Style.js').StyleFunction|undefined} Style
    */
   getStyleFunction() {
-    return undefined;
+    return this.styleFunction;
   }
 
   /**