[otrs] Creating a template issue

Hi Everyone, I followed the instructions on this website about creating your own theme http://doc.otrs.org/developer/2.0/en/html/c494.html The thing is that the fist step states that I need to create a new folder inside /html/ but Ubuntu is not letting me do because these folders were created by the owner "www-data". Does anyone knows why and how can I overcome this? Thanks in advance, Carlos