Arguments for/against theming and personalization of app

based on feedback from our customers, we would like to offer them options to personalize or at minimum theme the application. Some of us believe it is in the best interest of the customer to offer this functionality since the UX will improve, they will love working in the app and feel right at home. Some designers however are against this principle, since it would impact the experience of our brand & branding colours. Think of it as Outlook (one app for all) against Gmail (customize via theming).

Any ideas on this subject?

Why we should or should not listen to our users, or why we should keep our application with brand-colours ?