styles.xml

<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>


    <!-- Default Roboto styles, for all OS versions -->
    <style name="TextAppearance.Roboto" parent="TextAppearance.AppCompat">
        <!-- Setting the fontFamily to a non-null family causes TextView to use the system built-in fallback font -->
        <item name="android:fontFamily">sans-serif</item>
    </style>

    <style name="TextAppearance.Roboto.Normal">
        <item name="android:textStyle">normal</item>
    </style>

    <style name="TextAppearance.Roboto.Bold">
        <item name="android:textStyle">bold</item>
    </style>

    <style name="TextAppearance.Roboto.Italic">
        <item name="android:textStyle">italic</item>
    </style>

    <style name="TextAppearance.Roboto.BoldItalic">
        <item name="android:textStyle">bold|italic</item>
    </style>


    <!-- With < v21, Android will use the sDefaults array (i.e., Typeface.defaultFromStyle()) if and only if
         fontFamily=@null, typeface is omitted (defaults to -1 when parsed), and style > 0.

         But it gets trickier: when style == 0 (textStyle=normal), then you *must* also set typeface="sans"
         in order to trick it into using the Typeface.SANS instance.
      -->

    <!-- Default Lato styles -->
    <style name="TextAppearance.Lato" parent="TextAppearance.AppCompat">
        <!-- with fontFamily=null and typeface=normal, TextView will automatically
             fall back to Typeface.defaultFromStyle(int) based on the android:textStyle attribute,
             EXCEPT in the case of "normal", where we will also have to set typeface=sans -->
        <item name="android:fontFamily">@null</item>
        <item name="android:typeface">normal</item>
    </style>

    <style name="TextAppearance.Lato.Normal">
        <!-- with fontFamily=null and textStyle=normal, we can get the custom font by setting typeface=sans -->
        <item name="android:textStyle">normal</item>
        <item name="android:typeface">sans</item>
    </style>

    <style name="TextAppearance.Lato.Bold">
        <item name="android:textStyle">bold</item>
    </style>

    <style name="TextAppearance.Lato.Italic">
        <item name="android:textStyle">italic</item>
    </style>

    <style name="TextAppearance.Lato.BoldItalic">
        <item name="android:textStyle">bold|italic</item>
    </style>

</resources>