Both the Android and Windows apps have a dark theme and support dark mode. The iOS version doesn't have a dark theme yet, but it's a popular request! Dark themes are enabled based on your OS settings. The apps will match whatever theme you have set for your computer or phone.
Enabling dark mode on Windows
To enable the dark mode on Windows, see this article: https://www.pcmag.com/how-to/how-to-enable-dark-mode-in-windows-10
Enabling dark mode on Android
To enable the dark mode on Android, see this article: https://www.google.com/amp/s/9to5google.com/2018/12/17/android-dark-mode-theme-pie/amp/