Single web app, multiple client(UI localization)

I have one web application and multiple clients. Language localization is not an issue as it is already handled. But UI feature localization is now starting to look bad as the number of clients grows.

lets say I have 3 clients, A, B, C all the client want only small UI details or sometimes a bit bigger UI feature changed in single module. How I currently handle this is by creating localize directory for each of this clients and adding needed scripts there which allow me to add/change UI views. if A, B and C want button with different design then I have jquery trigger that listen to events in all of the localize directory. On production A client will only have localize_A directory and problem solved. But as the application grows bigger we have a lot of repeated code, dependency growing larger and larger. s you can imagine this is starting to get messy and not so easy to manage. thus we have decided to improve it even if it means refactoring big part of our project. I want to get rid of this and have good architecture to handle this issue. How would big project handle this kind of scenario?