it's just a hardcoded thing.
open template forumhome and below $navbar add:
HTML Code:
<center>
<select name="newstyleid" onChange="window.location='index.php?styleid=' + this.value">
<option value="0">Choose your vB.org style here:</option>
<option value="3">Dark Blue</option>
<option value="9">Admin CP Blue</option>
<option value="10">Admin CP Silver</option>
<option value="1">Purple</option>
<option value="2">Stylish Green</option>
<option value="6">Plain Green</option>
</select>
</center>
you have to change id's and stylenames