If you're on a G Suite team and use Google Chat on your phone, there's a sight for sore eyes if you're also a night owl: a dark theme is rolling out for both Android and iOS apps right now.
For Android 10 or later devices, the Dark mode toggle will be under the Display section of Chat's settings. If the toggle is on, the app will follow the system state. iOS users will need to hit the settings and then Theme. They can select either Light, Dark, or System default.
It may take up to 15 days before this gets out to every user.