Thank you for reporting this. This has been fixed for the upcoming UBB.threads 7.7.5.

While waiting for 7.7.5 to be released, the fix is rather simple to correct on your own forum.

FIND IN templates/default/admin/createforum.tmpl AROUND LINE 158
Code
<select name="stylesheet" class=formboxes>
<option value="usedefault" selected="selected">{$ubbt_lang['DEFAULT']}</option>

REPLACE WITH
Code
<select name="stylesheet" class="formboxes">
<option value="0" selected="selected">{$ubbt_lang['DEFAULT']}</option>

Current developer of UBB.threads PHP Forum Software
Current Release: UBBT 7.7.5 // Preview: UBBT 8.0.0
isaac @ id242.com // my forum @ CelicaHobby.com