カスタムUITableViewCellの選択時の背景色を変更する ref: https://qiita.com/k-yamada-github/items/a589fca3b68847bc3a99

file0.swift

// CustomCell.swift:

import UIKit

class CustomCell: UITableViewCell {

    override func awakeFromNib() {
        super.awakeFromNib()
        backgroundColor = UIColor(named: "primary")        
        selectedBackgroundView = makeSelectedBackgroundView()
    }

    private func makeSelectedBackgroundView() -> UIView {
        let view = UIView()
        view.backgroundColor = UIColor(named: "primary_light")
        return view
    }
}