private function bgStyles(array $colors) : void
    {
        if (count($colors) !== count($this->values)) {
            throw new Exception('Number of progress bar colours needs to match the number or progress bar values');
        }

        foreach ($colors as $color) {
            if (in_array($color, $this->supported_bg_styles) === true) {
                $this->bg_colors[] = $color;
            }
        }
    }