Select theme in Android Studio

You're only changing the Android Studio preview theme. to change the theme of the app must go to your AndroidManifest.xml file

and in the application tag it changes the theme parameter.

<application
        android:theme="@android:style/Theme.Translucent" >

This is just an example: To see other themes or create your custom you see the following link: link

In styles.xml, you'll see the theme used in your application, as in this example:

<resources>

    <!-- Base application theme. -->
    <style name="AppTheme" parent="Theme.AppCompat.Light.DarkActionBar">
        <!-- Customize your theme here. -->
        <item name="colorPrimary">@color/colorPrimary</item>
        <item name="colorPrimaryDark">@color/colorPrimaryDark</item>
        <item name="colorAccent">@color/colorAccent</item>
    </style>

    <style name="AppTheme.NoActionBar">
        <item name="windowActionBar">false</item>
        <item name="windowNoTitle">true</item>
    </style>

    <style name="AppTheme.AppBarOverlay" parent="ThemeOverlay.AppCompat.Dark.ActionBar" />

    <style name="AppTheme.PopupOverlay" parent="ThemeOverlay.AppCompat.Light" />

</resources>

See that the above code has colors that have been externalized in the "colors.xml" file, which is in the "values" folder, with the following code:

<?xml version="1.0" encoding="utf-8"?>
<resources>
    <color name="colorPrimary">#E76D00</color>
    <color name="colorPrimaryDark">#E76D00</color>
    <color name="colorAccent">#E76D00</color>

    <color name="colorTextView">#ff606060</color>
    <color name="colorTextViewDark">#ff020202</color>
</resources>

To edit the theme of your app, Android Studio provides a default theme editor - with the Android Studio IDE open, go to the menu tools > Android > Theme Editor and make changes as desired.