Описание тега android-theme
A theme is a style applied to an entire Android application or Activity, rather than an individual View. When a style is applied as a theme, every View in the Activity or application will apply each style property that it supports.
A theme is a style applied to an entire Android application or Activity, rather than an individual View. When a style is applied as a theme, every View in the Activity or application will apply each style property that it supports.
See also the official documentation.