How to turn on dark mode in iOS / iPadOS
First, you’re going to need to make sure you’ve updated your iPhone or iPad to iOS or iPadOS 13, at minimum. For the former, that means you’ll need at least an iPhone SE or iPad Air 2. To update your device to the most modern version of iOS you can get, select Settings app, select General, and select Software Update.
Once you have iOS / iPadOS 13 running, enabling your device’s dark mode is similarly simple. Select Settings > Display & Brightness, and then pick Light or Dark.
You can also use the “Automatic” setting to shift your device from its Light theme to its Dark theme at sunrise or sunset (or on whatever custom schedule you prefer). And if you find that some of your favorite apps don’t have a dark mode they can use, you can force that, too.
How to turn on dark mode in Android 9 or higher
You’ll first need to make sure you’re using the latest version of Android—specifically, Android 9, at minimum. For example, the Google Pixel stock version of Android, checking for and updating the smartphone involves selecting the Settings app, selecting System, selecting Advanced, and selecting System Update. (The specific steps / names of screens might vary depending on which Android you have; Samsung Galaxy S9 owners, for example, would go Settings > System Update > Check for Updates)
Once you are updated, go back to your Settings app. select Display, and you should either see an option for “Dark theme,” “Night mode,” or something to that effect. If you don’t, select the Advanced section, and you should find it there.
There’s no “schedule” feature for dark mode on Android; it’s either on or off. However, if you want to give yourself a little extra control over whether you’re staring at black or white backgrounds, swipe down from the top of your device, pull down the Quick Settings bar to expand it, and select the pen icon in the lower-left corner. From there, you’ll be able to add a dark mode shortcut, which at least gives you an easy way to switch modes.