There are two ways to create a custom theme style in android:
1. To an individual View, by adding the style attribute to a View element in the XML for your layout.
2. Or, to an entire Activity or application, by adding the android:theme attribute to the <activity>
or <application>
element in the Android manifest.
Please refer this
To set a theme for all the activities of your application, open the AndroidManifest.xml
file and edit the tag to include the android:theme attribute with the style name. For example:
<application android:theme="@style/CustomTheme">