I would mostly worry about compatibility issues with thirdparty addons, I have had mine with the otherwise beautiful (IMHO) Buyshop theme. A custom theme is also a budget issue if you cannot do it yourself.
It would be great if more themes from the Magento library would be ported to CS Cart of if you could ... learn your why in by attending a virtual classroom hosted by CS Cart (for a fee) on how to .. DIY a theme.
In general I have had good to very good experiences with Cart Power (Graceful), eCom Labs and ThemeHills. But in general I would prefer to go for development or customization of a theme to developers who also do addons and themes.