CSS stylesheet for making existing website design responsive

Hi, We are a web development company located in Zurich, Switzerland. We build web applications that we run ourselves. So far, our largest product is [login to view URL], an online survey platform that is already available in 12 languages and that is used all over the world. The visitor’s area of our website (when not logged in) is not yet optimised for mobile devices. However, we would like it to be responsive so that it can be viewed by all kind of devices. The design of the visitor’s area is separated in two slightly different styles: On the one hand, the main landing page ([login to view URL]) is centred and without borders. It contains an image slider in the top half. Please view attached print screen “[login to view URL]”. All subpages, on the other hand, contain a wrapper with a border (e.g. [login to view URL]). There’s no need to keep those borders in the responsive design. Please view attached print screen “[login to view URL]”. Thank you for your quote for the creation of a .css-file that we can include into our visitors area so that the whole design becomes responsive for all kind of screen widths (e.g. smaller than 960px). Best regards Lionel Marbot