An app is a special top-level component whose markup is in a .app
resource. Just like any other component, you can put CSS in its bundle in a resource called
<appName>.css.
For example, if the app markup is in notes.app, its CSS is
in notes.css.
When viewed in Salesforce1
and Lightning Experience, the UI components
include styling that matches those visual themes. For example, the
ui:button includes the
button--neutral class to
display a neutral style. The input components that extend
ui:input include the
uiInput--input class to
display the input fields using a custom font in addition to other styling.