I'm sorry but this doesn't have direct relation to wpForo. wpForo is loaded on a page. this is your theme's page layout, it can't be changed through wpForo. You should edit the page and find an option (theme option) to disable that background.
This also can be hidden using CSS code. Please leave the forum URL to allow us check it in case you didn't find such option in edit page screen.
yes i can edit every page on theme but forum is not editable on page thats why i asked and can i pm you the url instead? i dont want to share it yet