Is it possible to opt-out of dark mode on iOS 13?

According to Apple's session on "Implementing Dark Mode on iOS" (https://developer.apple.com/videos/play/wwdc2019/214/ starting at 31:13) it is possible to set overrideUserInterfaceStyle to UIUserInterfaceStyleLight or UIUserInterfaceStyleDark on any view controller or view, which will the be used in the traitCollection for any subview or view controller.

As already mentioned by SeanR, you can set UIUserInterfaceStyle to Light or Dark in your app's plist file to change this for your whole app.

First, here is Apple's entry related to opting out of dark mode. The content at this link is written for Xcode 11 & iOS 13:

Entire app via info.plist file (Xcode 12)

Use the following key in your info.plist file:

UIUserInterfaceStyle

And assign it a value of Light.

The XML for the UIUserInterfaceStyle assignment:

<key>UIUserInterfaceStyle</key>
<string>Light</string>

Apple documentation for UIUserInterfaceStyle

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Entire app via info.plist in build settings (Xcode 13)

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Entire app window via window property

You can set overrideUserInterfaceStyle against the app's window variable. This will apply to all views that appear within the window. This became available with iOS 13, so for apps that support previous versions, you must include an availability check.

Depending on how your project was created, this may be in the AppDelegate or SceneDelegate file.

if #available(iOS 13.0, *) {
    window?.overrideUserInterfaceStyle = .light
}

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Individual UIViewController or UIView

You can set overrideUserInterfaceStyle against the UIViewControllers or UIView's overrideUserInterfaceStyle variable. This became available with iOS 13, so for apps that support previous versions, you must include an availability check.

Swift

override func viewDidLoad() {
    super.viewDidLoad()
    // overrideUserInterfaceStyle is available with iOS 13
    if #available(iOS 13.0, *) {
        // Always adopt a light interface style.
        overrideUserInterfaceStyle = .light
    }
}

For those poor souls in Objective-C

if (@available(iOS 13.0, *)) {
        self.overrideUserInterfaceStyle = UIUserInterfaceStyleLight;
}

When set against the UIViewController, the view controller and its children adopt the defined mode.

When set against the UIView, the view and its children adopt the defined mode.

Apple documentation for overrideUserInterfaceStyle

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Individual views via SwiftUI View

You can set preferredColorScheme to be either light or dark. The provided value will set the color scheme for the presentation.

import SwiftUI

struct ContentView: View {
    var body: some View {
        Text("Light Only")
            .preferredColorScheme(.light)
    }
}

Apple documentation for preferredColorScheme

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Credit to @Aron Nelson, @Raimundas Sakalauskas, @NSLeader and @rmaddy for improving this answer with their feedback.