fix 404 : html-background.png ; git screen.css differs from web version

[leetbit]

get 404 from screen.css try to load html-background.png.
did local diff on the git and web version of screen.css.
only the the following line is affected:
https://github.com/CentOS/mirrorlists-code/blob/master/frontend/html/HEADER.images/screen.css#L73

background: white url(/HEADER.images/html-background.png);