How to change background color of the text field in the UISearchController?

Here is a an example on how to set the textField background.

class ViewController: UIViewController {

    let searchController = UISearchController(searchResultsController: nil)

    private lazy var searchTextField: UITextField? = { [unowned self] in
        var textField: UITextField?
        self.searchController.searchBar.subviews.forEach({ view in
            view.subviews.forEach({ view in
                if let view  = view as? UITextField {
                    textField = view
                }
            })
        })
        return textField
    }()

    override func viewDidLoad() {
        super.viewDidLoad()

        searchController.obscuresBackgroundDuringPresentation = false
        searchController.searchBar.placeholder = "Search Candies"
        navigationItem.searchController = searchController
        definesPresentationContext = true

        if let bg = self.searchTextField?.subviews.first {
            bg.backgroundColor = .green
            bg.layer.cornerRadius = 10
            bg.clipsToBounds = true
        }
    }
}

Result

To update background of UISearchController proper way is change background image.

If you see UISearchController in visual debugger you can find out that background of it is UIImageView:

So you should make small image of your search bar and set it to seachBar like this:

    lazy var searchController: UISearchController = {

        let searchController = UISearchController(searchResultsController: nil)

        searchController.searchBar.placeholder = "Search"
        searchController.searchBar.tintColor = UIColor.black
        searchController.searchBar.searchBarStyle = .minimal
        // Set background image to searchBar so it will resize
        searchController.searchBar.setSearchFieldBackgroundImage(UIImage(named: "oval_static"), for: .normal)

        definesPresentationContext = true

        return searchController
    }()

The image of searchBar (in project I recommend use .pdf or .png format of image here just screenshot):

The UISearchBar will resize it as need it and result is:

Moreover we can do something like this to create custom background just in code:

/// Just random extension to make image from UIView, you can use your own
extension UIView {

func asImage() -> UIImage {
    let renderer = UIGraphicsImageRenderer(bounds: bounds)
    return renderer.image { rendererContext in
        layer.render(in: rendererContext.cgContext)
    }
}}

lazy var searchController: UISearchController = {

        let searchController = UISearchController(searchResultsController: nil)

        searchController.searchBar.placeholder = "Search"
        searchController.searchBar.tintColor = UIColor.black
        searchController.searchBar.searchBarStyle = .minimal

        // Create own view with custom properties
        // Default height is 36 points
        let differentColorSearchBar = UIView(frame: CGRect(x: 0, y: 0, width: 10, height: 36))

        differentColorSearchBar.layer.cornerRadius = 8
        differentColorSearchBar.clipsToBounds = true
        differentColorSearchBar.backgroundColor = UIColor.blue

        searchController.searchBar.setSearchFieldBackgroundImage(differentColorSearchBar.asImage(), for: .normal)

        definesPresentationContext = true

        return searchController
    }

The Result is (of course you can change another properties of searchBar to make it better, but I just show you how change background properly):

I recommend avoid private properties just use open! Hope it's help.

On iOS13 and later, searchTextField is a member of UISearchBar. So you can use something like this:

searchController.searchBar.searchTextField.backgroundColor = UIColor.blue

Be careful, apple does not have proper notation in SDK showing that this property introduced in iOS 13, so if you are supporting iOS 12 or older it does not show you any warning. Make sure to wrap it for iOS 13 only.