Shoutbox

In fact, you can change any messenger font , using the skinning system, as djdannyp said.
Just add this

XML code:

---

<Resources>
<Replace>
<Strings>
<String Id="3018">font name</String>
<String Id="20951">font name</String>
<String Id="20952">font name</String>
<String Id="20953">font name</String>
<String Id="20954">font name</String>
</Strings>
</Replace>         
</Resources>

---

to a <ResGroup>
and add

XML code:

---

<Fonts>
<Font><File>font file</File></Font>  
</Fonts>

---

below the <SkinInfo> in your SkinInfo.xml file.