In Semplice v7 this option is now embedded into the navigation itself so it doesn't have to be set for every post individually.
By default, the content you create will appear after the navigation. If you remove your navigation from the page/project you will see the 70px white space on top of your content. Also, if you set your navigation to have a transparent background when you preview your page you will see the white background color.
In order to move your content on top, open your page editor and click "Look & Feel", find "Display Content" option and set it "Straight on Top". Then "Save" and "Publish" your changes.