Add full preview for the custom theme page

To have a consistent custom theme experience, I think user had to finish the custom theme flow(click the apply button in the naming page), so we don't support apply button for a undefined theme in the custom theme *full preview* page.
Video: https://drive.google.com/file/d/1oIGb5QTliCJYNEW6b6b9IttD8Mc8QJdE/view?usp=sharing

Test: Manually
Bug: 151285476
Change-Id: I1cad75f78b02c1dc87b55488b15ebdfbe85570a1