Well, I don't even think you need to buy anything. Basically copying existing skin you like (e.g., "classic" or "larry") to a new folder, changing the logo image, maybe also changing a couple of things here and there to get it working (you would probably need to consult with Roundcube skin development/installation guide), and then switching to it should be enough. You would also probably need to set the new skin as a default one.
Please note that there is a small chance the custom skin will partially break on update, but it won't be overwritten on update.