The application can - via API - add class names to controls which can be used to style those controls differently without changing the renderers or implemantation.
This button gets the additional class "customRedButton" and the application itself defines some style (red text and border) which is applied regardless of the current theme:
This button gets the additional style class name "sapUiBtnNext". Some themes (Platinum and Tradeshow Plus) define a distinct look for this type of buttons:
The following buttons show how additional style classes can be applied and removed dynamically and how they co-exist with other states and state changes: