Impact of UI customizations in EPBCS

We would like to customize the EPBCS userinterface like Icons, look and feel of the application , in terms of brandingexcept the already available options like image upload and background changes,would like to perform deeper customizations. For this we would like tounderstand the impact if we perform customization using JET framework and ifany oracle patch upgrades are applied what would be the impact of thesecustomizations? Also would like to understand how far we can proceed with UIcustomizations?