Whether you're using an Android phone or an Assistant-enabled speaker, most of what you do is saved by Google. This is a great way of checking what your speaker understood when it gave you an awkward answer, but also a rather disturbing way of seeing all the apps you've been using on your phone on a given day. Because it's usually more comfortable to check that history on a desktop computer, Google has given its My Activity page a dark mode option, making it easier on the eyes and helping you save battery.
Both My Activity's mobile and desktop versions are available in dark mode, with the option to automatically select it based on your system's setting. Interestingly, there is no option to manually change this setting, so you'll have to set your phone's or computer theme to dark.
If you're worried about the data Google tracks, rest assured you can delete your history and control what's saved in your Activity Control settings.