To create or import a theme, first go to the Theme module (at Global or Event level).
Then select whether you want to create a theme for websites (used for pages and forms), emails, or for the engagement app. Click on the plus button to create a new one.
Give your theme a name, paste your CSS code if you have already written it externally, and assign it to a team.
You can always change your CSS by clicking on your theme and editing it in the Theme tab.