Ciao, Stefano.
Sorry for the delay, but I was not notified of your answer.
I was using the Theme-1. But since then I’ve learned to customize the themes and I changed Theme-1 to a new one where I forced the font-family definition.
The excerpt row is now like this:
<tr>
<td style="font-size: 14px; color: #666; font-family: Arial, Helvetica, sans-serif">
<?php if ($image != null) { ?>
<img src="<?php echo $image; ?>" alt="picture" align="left"/>
<?php } ?>
<p><strong><a target="_tab" href="<?php echo get_permalink(); ?>" style="font-size: 18px; color: #000; font-family: Tahoma, Geneva, sans-serif; text-decoration: none"><?php the_title(); ?></a></strong></p>
<?php the_excerpt(); ?>
</td>
</tr>
And it’s working fine for me!
Anyway, I guess a css would be better.