In PowerApps, we can access Advanced Customization features. This will look familiar to Dynamics 365 developers and administrators who go into Settings in the Dynamics 365 app.
To do this in PowerApps, log into https://powerapps.microsoft.com: and select Settings -> Advanced Customizations:
This will open the Advanced Customizations pane:
From here, we can manage Users, Security Roles, Themes, Publishers and Developer Resources.
Users takes us to:
Note the Themes link took me to the app home page. I suspect this is a work in progress.