The ionic framework provides very beautiful UI Components like custom buttons, select boxes, loaders, alerts, toasts, checkboxes & radio, etc.
These are fully customizable with many options like for example if we take a button, we can easily change its color, shape, fill style with various available options given on respective UI component docs like for button it here
But for custom branding and modification of an application to a specific style may need deep style management of each and every UI component. The ionic framework is really a great platform that makes customizations and styling of Ionic web components very easy.