Use Nickel Theme for Only One Category

I'm hoping to use the Nickel Theme for only Used/Demo Equipment on the http://www.fairviewhifi.com website. It appears that this can be done after reading the following forum post: http://stackideas.com/forums/assign-a-custom-theme-template-to-a-category

Is there a line I can add to the appropriate file to make this happen?

Thanks,

Steve