How to set default font family for entire Android app

The answer is yes.

Global Roboto light for TextView and Button classes:

<style name="AppTheme" parent="AppBaseTheme">
    <item name="android:textViewStyle">@style/RobotoTextViewStyle</item>
    <item name="android:buttonStyle">@style/RobotoButtonStyle</item>
</style>

<style name="RobotoTextViewStyle" parent="android:Widget.TextView">
    <item name="android:fontFamily">sans-serif-light</item>
</style>

<style name="RobotoButtonStyle" parent="android:Widget.Holo.Button">
    <item name="android:fontFamily">sans-serif-light</item>
</style>

Just select the style you want from list themes.xml, then create your custom style based on the original one. At the end, apply the style as the theme of the application.

<application
    android:theme="@style/AppTheme" >
</application>

It will work only with built-in fonts like Roboto, but that was the question. For custom fonts (loaded from assets for example) this method will not work.

EDIT 08/13/15

If you're using AppCompat themes, remember to remove android: prefix. For example:

<style name="AppTheme" parent="Theme.AppCompat.Light.DarkActionBar">
    <item name="android:textViewStyle">@style/RobotoTextViewStyle</item>
    <item name="buttonStyle">@style/RobotoButtonStyle</item>
</style>

Note the buttonStyle doesn't contain android: prefix, but textViewStyle must contain it.

With the release of Android Oreo you can use the support library to reach this goal.

1. Check in your app build.gradle if you have the support library >= 26.0.0
2. Add "font" folder to your resources folder and add your fonts there

3. Reference your default font family in your app main style:

   <style name="AppTheme" parent="Theme.AppCompat.Light.NoActionBar">
      <item name="android:fontFamily">@font/your_font</item>
      <item name="fontFamily">@font/your_font</item> <!-- target android sdk versions < 26 and > 14 if theme other than AppCompat -->
   </style>
   

Check https://developer.android.com/guide/topics/ui/look-and-feel/fonts-in-xml.html for more detailed information.