Just tested it, it seemed so, anyone that you added as admins in your project, will also have the access to the Custom CSS field in the project page’s theme editor.
As long as the project is created under the user who has the Custom CSS enabled.
From what I experienced, if its the otherwise: a user who doesn’t have Custom CSS enabled creates a project, and then adds a user who has Custom CSS enabled, both user won’t have access to the project page’s Custom CSS. But this was quite a long time ago, maybe its different now.