Hi Jessica,
You have six places where you can place your CSS, why do you need an extra one? Can you explain better what your need is so we can help you better?
In any case, to add CSS to 5. Screen, you can add it on the Style Sheet property in the Screen you want the CSS to be applied:
To add the CSS to 6. Inline, you just need to select the element (in my case an Expression) you want to apply the CSS, select the tab Styles and add your CSS directly:
Regards,
João