class ThemesVC: UIViewController {

    var images: [UIImage] = []

    override func viewDidLoad() {
        super.viewDidLoad()

        images = ThemeManager().themeBackgroundImages()

        // whatever else you're doing in viewDidLoad
        // ...
    }

}

extension ThemesVC: UICollectionViewDelegate, UICollectionViewDataSource, UICollectionViewDelegateFlowLayout{

    func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
        return images.count
    }

    func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
        let cell = collectionView.dequeueReusableCell(withReuseIdentifier: "ThemesCollectionViewCell", for: indexPath) as! ThemesCollectionViewCell

        cell.contentView.layer.cornerRadius = 20

        cell.themeCellImageView.image = images[indexPath.row]

        return cell
    }

    func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, insetForSectionAt section: Int) -> UIEdgeInsets {
        return UIEdgeInsets(top: 0, left: 20, bottom: 0, right: 20)
    }

    func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, minimumLineSpacingForSectionAt section: Int) -> CGFloat {
        return 20
    }

    func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, minimumInteritemSpacingForSectionAt section: Int) -> CGFloat {
        return 20
    }

    func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, sizeForItemAt indexPath: IndexPath) -> CGSize {
        return CGSize(width: view.frame.size.width/2.34, height: view.frame.size.height * 0.3)
    }
}