Primer on Styles and Themes

styles_themes.xml

<!-- A theme. -->
<style name="AppTheme" parent="Theme.AppCompat.Light.DarkActionBar">
        <item name="colorPrimary">@color/colorPrimary</item>
        <item name="colorPrimaryDark">@color/colorPrimaryDark</item>
        <item name="colorAccent">@color/randomRed</item>
        <item name="alertDialogTheme">@style/MyDialog</item>
    </style>

<style name="MyDialog" parent="ThemeOverlay.AppCompat.Dialog.Alert">
        <item name="colorAccent">@color/secondary</item>
    </style>

<!-- Style -->
<style name="MyStyle">
        <item name="android:background">@color/red</item>
        <item name="android:alpha">0.5</item>
        <item name="android:maxLines">2</item>
    </style>

<!--Apply a theme inside manifest-->
<activity
            android:name=".MainActivity"
            android:label="@string/app_name"
            android:theme="@style/AppTheme"/>

<!-- Apply a style on a view-->
<TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Hello World!"
        style="@style/MyStyle" />