Custom CSS not working

When I open my list logged in, it appears. However, when I open my list logged out, it doesn’t work at all. Why is this? I want my iframed list to look like it’s part of the parent page.

The custom CSS only applies you, since it’s in your preferences.

You can choose a theme for a shared link, I think that’s the closest to what you want to do.

How can you do that?