Depends on what you class as "straightforward" - as you will need all the various scripts etc required to accept the items into the cart and then process the order it is probably best to use one of the clean CSS based template and rebuild the main Act_Primary around your exisiting page layout in terms of navigation, headers etc. You can then modifiy the Act_ProductLine templates to make these fit your desired layout.
I would probably remove the Actinic navigation in favour of your own and build in a "check out" button into your exisiting page designs.
With a bit of tinkering of the main templates there is no reason why you cannot integrate Actinic into your exisiting site relatively seemlessly
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