Page and form appearance is based on the Bootstrap v3.3.2 CSS framework. To define the appearance of a page or form you can use an application theme resource, apply a Boostrap theme, or manual add a custom CSS class that you implement.
Apply an application resource theme
By default, pages designed with UI designer are deployed in an application and use the theme chosen for the application.
Apply a Boostrap CSS theme
Many Bootstrap CSS themes are available (for example at https://bootswatch.com/).