In Manifest theme sets with style name (AppTheme and myDialog)/ You can set new styles in styles.xml
<intent-filter>
<action android:name="android.intent.action.MAIN" />
<category android:name="android.intent.category.LAUNCHER" />
</intent-filter>
</activity>
<activity
android:name=".MyActivity2"
android:label="@string/title_activity_my_activity2"
android:theme="@style/myDialog"
>
</activity>
</application>
styles.xml example
<resources>
<!-- Base application theme. -->
<style name="AppTheme" parent="android:Theme.Black">
<!-- Customize your theme here. -->
</style>
<style name="myDialog" parent="android:Theme.Dialog">
</style>
In parent you set actualy the theme