Google Assistant had been reported to gain a dark theme at the start of 2019. However, this update did not get to all users at once. Now, it appears that the Mountain View giant is pushing this change again. This new roll-out, which also affects the Google App, has been observed in a small number of countries thus far.
https://www.notebookcheck.net/The-Google-Assistant-and-App-is-seeing-a-roll-out-of-dark-theme-again.434844.0.html