how can the UI code of a product be kept a step away from the customers cutomisation?

im a UI deleloper for an educational product. How can the UI code of a product be kept a step away from the customers cutomisation? Now i m keeping the product branch and client's brach seperalty. But its really a headache to when its comes to issue fiexes.. need to update in all branches. Is any techinique to keep the product branch seperatly and client will overlide the customisations using webpack or any other technique.