How to use the level/config.themes function in level

Background.prototype._init = function () {
        var game = this.game;

        var imageName = 'bg-' + this.index;
        var image = game.add.tileSprite(
            0, 0,
            game.cache.getImage(imageName).width, game.height,
            imageName
        );
        this.image = image;
        image.tilePosition.x -= config.themes[this.index].offset;
        // image.alpha = 0.8;
    };

Midground.prototype._init = function () {
        var game = this.game;

        var imageName = this.imagePrefix + '-' + this.index;
        var image = game.add.tileSprite(
            0, 0,
            game.cache.getImage(imageName).width, game.height,
            imageName
        );
        this.image = image;
        image.tilePosition.x -= config.themes[this.index].offset;
        // image.alpha = 0.8;
    };