You installed the theme INCORRECTLY! UGH!
Read the README.txt file. It says:
Upload Pack-GorbGreenII/themes/GorbGreenII fodler to your themes folder on your FTP.
THAT IS ALL!
Looks like you tried to add it to the Forums style area. w:

My Advice.
Go into your MySQL Database.
Go to your _bbconfig table and browse it. Under there is a value called default_style. Change it's number to 1 instead of whatever it is. The number for the default sttyle needs to be the number that is set for the subSilver template in the _bbthemes table, which is usually 1, unless you deleted that Style from the Forums Admin Styles area.