Redesign an existing app is a big challenge. Today you have basically only the choice to change to a web application, because if you have a lot of views, forms and pages and have to update each design element separately - no customer will pay for it. One Option is to use xPages (for Notes Client and/or browsers), but it takes also a lot of time to implement in existing business applications.
Additional benefit: users are more comfortable if they can choose something, e.g. between different color schemas.
It would be very helpful if existing Notes native design elements as forms, views, folders, ... can be fully designable by CSS (or something similar), including e.g. text field properties or date time.