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

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

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

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
}

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

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

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

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.

If you are not using Xcode 11 or later (i,e iOS 13 or later SDK), your app has not automatically opted to support dark mode. So, there's no need to opt out of dark mode.

If you are using Xcode 11 or later, the system has automatically enabled dark mode for your app. There are two approaches to disable dark mode depending on your preference. You can disable it entirely or disable it for any specific window, view, or view controller.

Disable Dark Mode Entirely for your App

You can disable dark mode by including the UIUserInterfaceStyle key with a value as Light in your app’s Info.plist file.

This ignores the user's preference and always applies a light appearance to your app.

Disable dark mode for Window, View, or View Controller

You can force your interface to always appear in a light or dark style by setting the overrideUserInterfaceStyle property of the appropriate window, view, or view controller.

View controllers:

override func viewDidLoad() {
    super.viewDidLoad()
    /* view controller’s views and child view controllers 
     always adopt a light interface style. */
    overrideUserInterfaceStyle = .light
}

Views:

// The view and all of its subviews always adopt light style.
youView.overrideUserInterfaceStyle = .light

Window:

/* Everything in the window adopts the style, 
 including the root view controller and all presentation controllers that 
 display content in that window.*/
window.overrideUserInterfaceStyle = .light

Note: Apple strongly encourages to support dark mode in your app. So, you can only disable dark mode temporarily.

Read more here: Choosing a Specific Interface Style for Your iOS App

********** Easiest way for Xcode 11 and above ***********

Add this to info.plist before </dict></plist>

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

Xcode 12 and iOS 14 update. I have try the previous options to opt-out dark mode and this sentence in the info.plist file is not working for me:

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

Now it is renamed to:

<key>Appearance</key>
<string>Light</string>

This setting will block all dark mode in the full app.

EDITED:

Fixed typo thank you to @sarah