In TableHeaderRow, there is such a method.
    protected NestedTableColumnHeader createRootHeader() {
        return new NestedTableColumnHeader(tableSkin, null);
    }

When subclass wants to override it, it doesn't have access to the tableSkin because it is a private field. I recommend changing signature to

    protected NestedTableColumnHeader createRootHeader(TableViewSkinBase tableSkin) {
        return new NestedTableColumnHeader(tableSkin, null);
    }

Or provide a getter for the tableSkin

    protected TableViewSkinBase getTableSkin() {
        return this.tableSkin;
    }