The background is defined in the css . To change it you have to replace the file "page-bg.jpg" located in the folder "extension/ezflow/design/ezflow/images".
I think you need change tag body into file CSS (site-colors) address: \htdocs\vets\var\storage\packages\eZ-systems\ezwebin_design_blue\files\default\file .
But my address is demo . I'm using ezpublish 4.4.0