FontChangeCrawler.java

public class FontChangeCrawler
{
    private Typeface typeface;

    public FontChangeCrawler(Typeface typeface)
    {
        this.typeface = typeface;
    }

    public FontChangeCrawler(AssetManager assets, String assetsFontFileName)
    {
        typeface = Typeface.createFromAsset(assets, assetsFontFileName);
    }

    public void replaceFonts(ViewGroup viewTree)
    {
        View child;
        for(int i = 0; i < viewTree.getChildCount(); ++i)
        {
            child = viewTree.getChildAt(i);
            if(child instanceof ViewGroup)
            {
                // recursive call
                replaceFonts((ViewGroup)child);
            }
            else if(child instanceof TextView)
            {
                // base case
                ((TextView) child).setTypeface(typeface);
            }
        }
    }
}

Handle Adapters, etc.

Replacing Activity fonts goes a long way, but most of us also have a plethora of ListViews. The list items in a ListView are built within an adapter, not within an Activity. Therefore, you will also need to use the FontChangeCrawler in your adapters:
if(convertView == null)
{
    convertView = inflater.inflate(R.layout.listitem, null);
    fontChanger.replaceFonts((ViewGroup)convertView);
}
...
What's Not Handled?
I will leave handling the ActionBar as an exercise for the reader. Also, consider how you might handle widgets whose typeface you don't want to change.

Replace Activitys Typeface

@Override
public void setContentView(View view)
{
    super.setContentView(view);

    FontChangeCrawler fontChanger = new FontChangeCrawler(getAssets(), "font.otf");
    fontChanger.replaceFonts((ViewGroup)this.findViewById(android.R.id.content));
}


If you are not familiar with android.R.id.content, it is the official ID given to the root View within an Activity's layout.

Consider placing the above logic in a BaseActivity class

Replace Fragment's Typeface

@Override
public void onActivityCreated(Bundle savedInstanceState)
{
    super.onActivityCreated(savedInstanceState);

    FontChangeCrawler fontChanger = new FontChangeCrawler(getAssets(), "font.otf");
    fontChanger.replaceFonts((ViewGroup) this.getView());
}

You will need to apply the FontChangeCrawler to each Fragment as well. Consider placing this logic in a BaseFragment class.