Final answer:
The AndroidManifest.xml file declares the theme for an Android application using the application element and the android:theme attribute.
Step-by-step explanation:
The AndroidManifest.xml file uses its application element to specify the theme for the application. When you define an Android application, this file serves as a central location where you can declare the configuration of the application environment.
It is within the application element of this file that developers set the overall theme of the application by using the android:theme attribute.
This controls the visual style for the application, which is applied to all the activities, unless overridden on a per-activity basis.