If you want to style the subtitle also then simply add this in your custom style.
<item name="android:subtitleTextStyle">@style/MyTheme.ActionBar.TitleTextStyle</item>
People who are looking to get the same result for AppCompat
library then this is what I used:
<?xml version="1.0" encoding="utf-8"?>
<resources>
<style name="CustomActivityTheme" parent="@style/Theme.AppCompat.Light.DarkActionBar">
<item name="android:actionBarStyle">@style/MyActionBar</item>
<item name="actionBarStyle">@style/MyActionBar</item>
<!-- other activity and action bar styles here -->
</style>
<!-- style for the action bar backgrounds -->
<style name="MyActionBar" parent="Theme.AppCompat.Light.DarkActionBar">
<item name="android:background">@drawable/actionbar_background</item>
<item name="background">@drawable/actionbar_background</item>
<item name="android:titleTextStyle">@style/MyTheme.ActionBar.TitleTextStyle</item>
<item name="android:subtitleTextStyle">@style/MyTheme.ActionBar.TitleTextStyle</item>
<item name="titleTextStyle">@style/MyTheme.ActionBar.TitleTextStyle</item>
<item name="subtitleTextStyle">@style/MyTheme.ActionBar.TitleTextStyle</item>
</style>
<style name="MyTheme.ActionBar.TitleTextStyle" parent="@style/TextAppearance.AppCompat.Widget.ActionBar.Title">
<item name="android:textColor">@color/color_title</item>
</style>
</resources>