Google Photos is the last application from Google to receive the dark theme treatment. The company has started rolling out a dark theme to the Photos app, though do note that this is a server-side change. That basically means that you'll get it when you get it, you don't need to download a specific update from the Play Store, though having the app updated to the latest version will certainly help.
Interestingly enough, you will not get a dark theme switch in the Google Photos settings, no, as the activation of Google Photos' dark theme basically depends on your system's theme option. If your Android device is set to a dark theme in general, Google Photos app will eventually become dark.
This does not apply only to Pixel users, by the way, it seems like various devices already received this feature, including both OnePlus and Samsung devices. It does seem like your device needs to be running Oreo or Pie, though.
Google Photos' dark theme has not become available to our devices just yet, but you can check out what it will look like once it arrives in the gallery down below, courtesy of 9to5Google. As you can see, Google opted to include a dark gray background in the Photos app, instead of opting for a completely black background. This is a similar approach the company took with a number of other applications, in fact.
The status bar becomes dark gray as well when you're using the app, and the bottom bar as well, though the bottom bar is slightly darker than the app's background. This look is active throughout the app.
Google had already rolled out a dark theme to a number of its applications, including Messages, Calendar, Phone, YouTube, and so on. In fact, Google Calendar and Google Keep (also known as Keep Notes) applications received the dark theme treatment quite recently. In both of those applications, you are getting a manual switch, though, in the settings. So, it is possible that such a switch will come to the Photos app as well, we'll see.
Google Photos doesn't seem to be the only app that is receiving a dark theme, as some users received the same treatment in Google Drive. The dark theme appeared in Google Drive for me twice thus far, but on both occasions, it disappeared the moment I left the application. It seems that Google has been testing the dark theme in the app, and it may be ready to roll it out now, as some users already received it.
Google is on full offensive when it comes to dark themes, as a ton of its applications received the dark theme treatment in the last couple of months. Google realized that dark backgrounds and dark elements on the screen in general help out battery life on smartphones with OLED displays, which is one of the main reasons we're getting the option.
It is not yet known when will dark theme arrive for Gmail, Hangouts, and several other Google-made apps, but it would be immensely useful in Gmail in specific. Chrome already received a dark theme, though it is not yet a full-fledged feature, as it needs to be enabled through developer settings. Google is expected to make a dark theme available via Chrome's settings in the near future, though.