The user interface (UI) is rendered using the active theme and UI elements: form, table, actionbar, menu, etc.. The UI elements are independent of themes and are embed into the application. Designers and developers can make use of the standardized UI elements to offer a consistent experience across the application.
The UI elements are constructed using PHP syntax and are completely separate from the layout. As a designer you may have to define the template for certain UI elements but they are not embed into themes. The UI elements can be manipulated using PHP before rendering or using CSS/jQuery after being rendered.