Open the page you want fix up in Dreamweaver
On the Design panel be sure the CSS STYLES tag is selected. Click the add style button ( a + over a sheet at the lower right) use these settings:
Click OK and use these settings (choose your picture)
Click on the BODY tag at the bottom of your page, in the drop-down style option choose your new style to apply it to the body of the page (meaning the entire page), THe page will look odd in Dreamweaver but click F12 to preview it as a web page and it will look good like this one.