Custom font to whole activity

public class ReplaceFont {

public static void changeDefaultFont(Context context, String oldFont, String assetsFont) {
    Typeface typeface = Typeface.createFromAsset(context.getAssets(), assetsFont);
    replaceFont(oldFont, typeface);
}

private static void replaceFont(String oldFont, Typeface typeface) {
    try {
        Field myField = Typeface.class.getDeclaredField(oldFont);
        myField.setAccessible(true);
        myField.set(null, typeface);
    } catch (NoSuchFieldException e) {
        e.printStackTrace();
    } catch (IllegalAccessException e) {
        e.printStackTrace();
    }
}

// Put your font to assets folder...

ReplaceFont.changeDefaultFont(getApplication(), "DEFAULT", "LinLibertine.ttf");