Hi,
Yes, you can style second level items in CMSTreeMenu. You just need to add prefix for level of menu you would like to style.
For example you may add CSS prefix: ;sub; which adds to second level prefix "sub". Then you can go to CSS stylesheet and add class like this:
.subCMSTreeMenuLink {
background: blue;
}
This will add blue background to second level of menu.
Best regards,
Vita Janecek