
I hope you understand the concept of style, so now let’s try to understand what that scene.
<application android:theme="@style/CustomFontStyle">
The theme is nothing but Android style applied to the overall activity or application instead of individual display.
<activity android:theme="@style/CustomFontStyle">
Thus, when the force is applied as a theme, every View in Activity or application will apply every style properties that it supports.
<style name="CustomTheme" parent="android:Theme.Light">
...
</style>
But if you want a theme that is applied only on one Activity in your application, then add android: theme only attribute to the <activity> tag only.
<resource>
<style name="AppTheme" parent="android:Theme.Material">
<item name ="android:color/primary">@color/primary</item>
<item name ="android:color/primaryk">@color/prima</item>
<item name ="android:colorAcent/primary">@color/accent</item>
</style>
<resource>

Source: MasterAndroid (App)