Navigation folders - using indices or banners on pages inside the folder?

Site URL: https://badger-flugelhorn-yp72.squarespace.com

Hello,

I've recently added a folder to my site navigation as such:

But I noticed that Index-type navigation can't be added inside the folder. I have been using the Index type navigation to control things such as banner, alternating background colors on different sections, etc. For example: 

I would like all of the pages including those within the folder to have a Banner at the top, and be able to alternate background colors in different sections like the Home page allows.

Is there a way to do this within a folder?

Site password is 123

Thank you!