Quite a few Google-made applications have the dark theme / mode support these days, as Google started pushing such an option to its apps due to changes in Android 9 Pie's design. The company started removing non-white colors from its apps, so white became predominant.
In order to counter that, and offer users a chance to save some battery life on their OLED displays, the company actually introduced dark mode in a number of its applications, and that option is coming to more apps in the future.
At the moment, dark mode is available for Google Photos, YouTube, Snapseed, Google Contacts, Google Calendar, Google Phone, Google Keep, (Android) Messages, Google Chrome, Google News, Gboard, Google Drive (for some users), Google Calculator, Google Play Games, Google Maps, and Google Authenticator.
The dark mode is expected to become available in Google Drive for everyone in the near future, while dark mode for Gmail is also on the way. Following that, the dark mode may roll out to the Play Store, and some other Google-made apps, but we still do not have any info on that.
Having said that, we'll list every single app with dark mode down below (in no specific order), along with a short explanation as to how to activate dark mode for that app. Before we start, though, do note that for some of these apps, such as Google Photos, you need to be running Android 8.0 Oreo or Android 9 Pie, at least for the time being. We were also not able to get the dark mode running in Google Discover Feed, which is why it's not included on this list.
In order to activate dark mode in Google Photos, you need to activate Developer Options (unless you have a Pixel phone or an Android One device, as such an option is located in general settings on those devices). Open the Settings app on your phone, and navigate to the "About phone" section. There, you will see your phone's "Build number", tap that a bunch of times until you get a toast message that the Developer options have been activated.
Once that is done, go back to the Settings app's main screen, and there you'll see "Developer options", if not, you need to look for it in a "System" menu. Once you find it, hit the Developer options section, and scroll the list until you find "Night mode". Once you tap that, select the "Always on" option. At that point simply leave Settings, and the dark mode should be active in Google Photos, if not, swipe away Google Photos card from the overview menu first.
Activating dark mode in YouTube is really simple. Open the app, and tap your profile picture in the top-right corner. There you will see the "Settings" option, hit that, and enter the "General" menu. There you will see a "Dark theme" option, simply flip the switch, and that's it.
Once you open Snapseed, you will see three vertical dots in the top-right corner of the app's UI, tap that, and select "Settings". There you will see the "Dark theme" option, tap it, and the dark theme will be immediately activated in the app.
Open the app, and then open the hamburger menu either by swiping from the left edge towards the center, or by tapping the three horizontal lines in the top-left corner of the app's UI. Once you're there, you'll see "Settings", tap that, and in the next menu, you will notice the "Choose theme" option. There you will be able to select a dark mode option that will be active all the time, or will be able to set the app to activate dark mode when battery saver is on.
Activating dark mode in Google Calendar is somewhat similar to doing it in Google Contacts. Once you open the app, open the hamburger menu, and select "Settings". Once you're in that menu, tap the "General" option, and you will see a "Theme" option there. You can set it to run at all times, or only when you activate battery saver.
Once you're in the Google Phone application, hit the three vertical dots in the top-right menu, and there you will see the app's Settings. Tap that, and then select "Display option". In that menu, you will notice the "Dark theme" option.
Google Keep aka Keep Notes comes with a manual switch for dark mode as well. Once you open the app, open the hamburger menu, and tap settings. There you will see the "Enable Dark Theme" option, flip the switch, and that's it.
Once you open (Android) Messages, you will notice three vertical dots in the top-right corner of its UI. Tap that, and in the pop-up menu, you will see the "Enable dark mode" option. As soon as you tap that option, dark mode will be activated.
Activating dark mode in Google Chrome is a bit more complicated than you may think, as you need to enter Chrome's developer options, essentially. Open the app, and in the search field type in "chrome://flags".
Once you do that, and the new menu opens, you will see "Search flags" option. In that field, type in "Android Chrome UI dark mode", and once you find the option, hit the "Default" settings, and from the pop-up menu select "Enabled".
Repeat the process, but this time, instead of "Android Chrome UI dark mode", search for "Android web contents dark mode". When you find it, select "Enabled" as well. At that point, go to the overview menu, and swipe away the Chrome multitasking card.
Launch the app once again, and tap the three vertical dots in the app's UI. From there, tap "Settings", and you will see the "Themes" option. Once you enter that menu, you will be able to select a "Dark" theme option.
Activating everything we mentioned here will activate dark theme both for Chrome's UI, and content that you load using the browser. Do note that this is not a stable feature just yet, so some of the images you load may end up being inverted.
You can keep loading web pages in their regular look by not enabling "Android web contents dark mode", but that will provide you with a dark mode for the app's UI only. It's your call.
Once you open the Google News app, you need to tap your profile picture in the top-right corner of the app's UI. There, you will see the "Settings" option, tap that, and in that menu, you'll notice a "Dark theme" option. You can enable it there, to run at all times, or simply select only at night or only when the battery saver option is selected, it's up to you.
Gboard doesn't have a dark mode per se, as you cannot enable a dark theme for the app's settings, at least not yet. Considering you will be looking at the keyboard itself 99-percent of the time, though, you can always select a dark theme option for the keyboard itself, as you can basically choose which color you want the keyboard to be. You will see such an option once you enter Gboard settings, as the "Theme" option is located in that menu.
Activating Google Drive dark theme will not work for everyone, at least not at the time of writing this article. The procedure is completely the same as for Google Photos, which means that you need to navigate to Developer Options (unless you have a Pixel phone or an Android One device, as such an option is located in general settings on those devices), and find a Night Mode there.
In Google Calculator, the dark theme option is located in settings. Hit the three vertical dots option in the top-right corner, then select "Choose theme" option. There you will be able to choose between Light, Dark, and Set by Battery Saver options.
Google Maps offers dark mode for navigation itself. So, this is not exactly an app-wide dark mode, but it's still quite useful, as you will be using actual maps in this app most frequently, not messing around in settings.
Once you open Google Maps, open the hamburger menu from the top-left corner, and access "Settings". There you will see "Navigation settings" option, and from there scroll down to "Map display". There you can change the "Color scheme" from "Automatic" to "Day" or "Night", if you select night, dark mode will be activated while you're using Google Maps' navigation.
Google Play Games
Activating dark theme in Google Play Games is fairly easy. Once you open the app, hit the three vertical dots in the top-right corner. From there, select "Settings" and you will notice the "Dark theme" toggle in that menu, flip the switch, and that's it.
The procedure to activate dark mode in Google Authenticator is fairly similar to doing it in Google Play Games app. Once you open the app, hit the three dots in the top-right corner, and there you will see "View in dark mode" option… tap that, and you're good to go.