Re-skinning a web site essentially dresses the same content in new packaging
Most re-skins are carried out using advanced CSS. The letters CSS stand for "Cascading Style Sheets" and these are text files which instruct browsers in the fine art of presenting the content of the web site the way the designer originally intended. When used well- confining presentation markup to the style sheets means that site-wide changes can be implemented with far less labour than if they were to be written manually, page by page. Imagine having to change the heading colour from blue to red on every single heading when the web site has a thousand pages!
It doesn't take much to realise that better planning in the design and development of a web site can make a huge difference to providing scope for easy adjustment later.