You should use the skin to specify the presentation of your content. You can style all presentation elements of your interface in the skin, including typography, borders, backgrounds, layouts, and UI placement and alignment.

Vinay Kamath, User Interface Product Manager, explains the right way to use skin to style your application.

By defining presentation attributes in the skin - and not in sections - you can separate content (which is defined in sections and harnesses), from its presentation (which is defined in the skin.) This ensures greater consistency and promotes reuse.

Beginning with 7.1, you can easily define a single skin for your application, ensuring that all portals are presented in the most consistent manner.

The skin comprises mixins and style formats.

• A mixin is a reusable typography, border, or background style, or combination of these styles, that you can use when defining other mixins and style formats for components.
• A style format defines appearance for components, such as dynamic layouts, or controls, such as buttons and links. You can define multiple style formats for each component.