CSS changes on existing layout

We need some css changes on an existing codeigniter application. We need to give the layout a more business and clean-cut feeling. You must have worked in web layouts in the past and you will have to send one or more interfaces that show professionalism and minimal elements. Please notice I am not asking for a web site but a "skin" for an existing codeigniter interface. Do propose how you can work with the existing installation, I can give you access to the existing environment. Attached is a screen of the existing layout.