[ic] Running 'Themes'

[In The Sun, LLC]

Hi all
I have seen a few posts recently about running separate "Themes" on one
Interchange Cart.  I am trying to run a 'public' theme and a 'private' theme
of one cart, where the public side would have one look and feel to the whole
shopping experience and the private side would have a different look and
feel to the design.
Has anyone done this and if so how can two different designs run off one IC
cart? Would this be done by the templates?  Then how can the 'checkout' and
order process work off two different templates?
I have seen a post that this can be done through separte CSS files.  But how
is this done?
I am not an advanced programmer of Interchange, so I may be missing
something here...

Thanks for any advice or help.
MDell