message: Make it possible to specify extra Css files in the Pop Down List Picker.
diff: @@ -62,6 +62,8 @@ class PopDownListPicker<T extends { id: string; }> extends ThemeableElementBase
  private _laterHandle: domutils.LaterHandle;
  + private _extraCssFiles: ThemeTypes.CssFile[];
  +
  private _initProperties(): void {
  this._entries = [];
  this._selectedId = null;
  @@ -71,6 +73,7 @@ class PopDownListPicker<T extends { id: string; }> extends ThemeableElementBase
  this._formatEntries = (filteredEntries: T[], selectedId: string, filterInputValue: string): string =>
  filteredEntries.map(entry => `<div ${PopDownListPicker.ATTR_DATA_ID}='${entry.id}'>${entry.id}</div>`).join("");
  this._laterHandle = null;
  + this._extraCssFiles = [];
  }
  geSelected(): string {
  @@ -130,6 +133,16 @@ class PopDownListPicker<T extends { id: string; }> extends ThemeableElementBase
  this._formatEntries = func;
  }
  + /**
  + * Specify extra Css files to load into this element.
  + *
  + * @param extraCssFiles extra Css files which should be loaded along side the default set.
  + */
  + addExtraCss(extraCssFiles: ThemeTypes.CssFile[]): void {
  + this._extraCssFiles = [...this._extraCssFiles, ...extraCssFiles];
  + this.updateThemeCss();
  + }
  +
  //-----------------------------------------------------------------------
  //
  // #
  @@ -199,7 +212,8 @@ class PopDownListPicker<T extends { id: string; }> extends ThemeableElementBase
  }
  protected _themeCssFiles(): ThemeTypes.CssFile[] {
  - return [ThemeTypes.CssFile.GUI_CONTROLS, ThemeTypes.CssFile.FONT_AWESOME, ThemeTypes.CssFile.GUI_POP_DOWN_LIST_PICKER];
  + return [ThemeTypes.CssFile.GUI_CONTROLS, ThemeTypes.CssFile.FONT_AWESOME,
  + ThemeTypes.CssFile.GUI_POP_DOWN_LIST_PICKER, ...this._extraCssFiles];
  }
  private _updateEntries(): void {
  