Not sure what you mean by 'dialogue' box. Do you mean the white background of the main content area? If so, look on like 197 of the theme's main.css file (theme_clearview/themes/clearview/main.css) for #content and make any changes there. That area does use an image for the transparent background. The image you would need to modify is located here: theme_clearview/themes/clearview/images/white-bg.png

Let me know if you have any other questions.