You can rename your styles to whatever you want them to be if they’ve been applied already to items. In the “Inspector” the “Title” input area is where things get named for Images and DIV ID’s (if you’re doing a CSS Layout, blue button on).
The luxury of Freeway is that if you change the name of the styles or the Image names in the title bar there and Freeway will keep those styles linked through the name change so you don’t have to reapply them, Freeway will republish those items and rename them the next time you publish your document.
You can also import an existing style-sheet by clicking on the Ay (styles palette) and hitting the cog in the lower left and there should be an import option.