Workaround for lack of `listRowSelectedBackground` or some way of defining an adaptive background style with your own color

SelectedBackgroundColorHackery.swift

struct ContentView: View {
    var body: some View {
      List {
         SomeRow()
             // set the background of the row
            .listRowBackground(ListRowBackground())
        
            // .listRowSelectedBackground(ListRowSelectedBackground())
            // Oh no, `listRowSelectedBackground` above is not a real API, so list cells don't highlight when selected anymore. 
            // What do we do now? 
            // Traverse the UIKit hierarchy and configure cells in it within `ListRowBackground`.        
      }        
    }
}

// A background view that traverses the hierarchy to setup the underlying List's UIKit UICollectionViewCell
@MainActor
private struct ListRowBackground: UIViewRepresentable {
    func makeUIView(context: Context) -> SelectableBackgroundView {
        let selectableView = SelectableBackgroundView()
        return selectableView
    }

    func updateUIView(_ selectableView: SelectableBackgroundView, context: Context) {
        // theme has styles 
        let theme = context.environment.theme
        selectableView.backgroundColor = UIColor(theme.groupedlListItemBackgroundColor)
        selectableView.selectedBackgroundColor = UIColor(theme.groupedlListItemSelectedBackgroundColor)
    }
}

private class SelectableBackgroundView: UIView {
    var selectedBackgroundColor: UIColor? = nil {
        didSet {
            if selectedBackgroundColor != oldValue {
                updateCellBackground()
            }
        }
    }

    override func didMoveToWindow() {
        super.didMoveToWindow()
        // the background view should be installed on the cell at this point
        updateCellBackground()
    }

    func updateCellBackground() {
        guard let cell = firstSuperview(of: UICollectionViewCell.self) else {
            return
        }
        let selectedBackgroundView = UIView()
        selectedBackgroundView.backgroundColor = selectedBackgroundColor
        cell.backgroundColor = backgroundColor
        cell.selectedBackgroundView = selectedBackgroundView
    }
}

// hierarchy-traversal utility
extension UIView {
    func firstSuperview<T: UIView>(of clazz: T.Type) -> T? {
        guard let superview else { return nil }
        if superview.isKind(of: clazz) {
            return superview as? T
        }
        return superview.firstSuperview(of: clazz)
    }
}