are you creating a new page with any page template? you try to select blank instead of any page template,
Yes by default it will give you white background, you can override the same with your own css file,
I create a jspx page in WebCenter Portal Application as a blank page.
After create i add it to page hierachy and run application without doing anything to this page.
when it executed it isn't white whole the page. It was gradient form white at the top to light blue at the bottom.
How can i do it to white whole page?
The default skin has an entry for including both a background color and a background image for the af|document style selector. You can override this by adding in the following to the default portal-skin.css: