As far as I'm aware, you can't declare custom fonts in xml or themes. I usually just make custom classes extending textview that set their own font on instantiation and use those in my layout xml files.
ie:
public class Museo500TextView extends TextView {
public Museo500TextView(Context context, AttributeSet attrs) {
super(context, attrs);
this.setTypeface(Typeface.createFromAsset(context.getAssets(), "path/to/font.ttf"));
}
}
and
<my.package.views.Museo900TextView
android:id="@+id/dialog_error_text_header"
android:layout_width="190dp"
android:layout_height="wrap_content"
android:gravity="center_horizontal"
android:textSize="12sp" />