Hey @wonka! As far as I know, custom CSS still needs to be applied at the app level in the Custom CSS section of the Advanced Settings tab.
But, you don't have to style each container individually using substring matching, which I have a detailed post on here. Essentially, as long as the name for each container in your app starts with ācontainerā like they always do by default (i.e. container1, container2, etc.), you can target them all at once by using the following in place of what you have if applied in the Custom CSS section in the Advanced Settings tab: