You can change the padding in:
/* Main forum area. */
#wrapper {
padding: 1.2em 1em;
margin-top: 1em;
border: 3px solid;
border-radius: 5px;
}
Something like:
/* Main forum area. */
#wrapper {
padding: 1.2em 1em 5em 1em;
margin-top: 1em;
border: 3px solid;
border-radius: 5px;
}
would do.
Adapt the 5em to whatever you want.