Enable Dark mode in Photos app
Open Photos app in Windows 10. Click on the three-dotted button visible in the top-right corner and select Settings.
Scroll down a bit until you get Appearance tag. Under that, you will see Mode. By default, it should be set to Light. You will find two other options Dark and Use system setting. If you need a dark mode, simply choose Dark and restart the Photos app.
If you are using Windows 10 v1809, you will see the Use system setting option. If you choose this option, it will start using the same theme as you have in Windows Settings panel. If you have enabled Dark Mode in Windows Settings, you will get dark mode in Photos app and vice versa. You can enable dark mode in Windows Settings, Microsoft Edge, Twitter app, Movies & TV app, etc. too.
