Your using the wrong export option in style admin.
I do not exactly use Invisionboard but I remember on working with people's installs over the years there is an option to save css to files in the style_images directory. You might want to ask Invisionboard support how you operate that option. We look for the style on disk so css_6.css must exist in the style_images directory.
Since your operating on a localhost and not on the web where I can assist you in how to operate your forum your only recourse would be to ask Invisionboard how to operate their program