A theme defines the visual layout with a consistent look and feel across all forms in a site. You can configure a theme with a font style, font color, and page color. Also, you can create a theme by associating a CSS (cascading style sheet) and selecting a style for basic controls, such as buttons and check boxes, and for tables, tabs and menus.

See Create a theme.

Once a theme is associated with a site, the theme is automatically applied to all the forms within the site. However, you can use the extended properties for any form control to override the theme.

See Form controls.

Any settings for the controls at the form level override the site-level settings in the following order: control-specific styles, style sheet used in theme, and General settings. When you modify a theme, all forms are automatically updated.