Basic apps have a property, Theme, that defines the built-in theme which is applied to borders, toolbars and backgrounds for the App Framework:

See Themes for an example of the areas affected when basic apps are published individually. When basic apps are included in workspace apps, the workspace overrides the themes of individual apps within the workspace.

You can change the theme assigned to a basic app using any of the valid values for this property:

You can also customize themes. See Customize Themes for Basic Apps and Views for more information.

See also App styling in Edit App Properties/Themes.