The design mode allows you to edit and design your website's layout. Make your website more attractive by adding video backgrounds, background colors, pictures, texture (and etc) with the tools for the Background and Body layers.
Each layer gives you the ability to design a different area of your website.
Any changes you make under the design mode will be applied to the particular selected area on the whole website just like a global CSS.
The Background has one set of tools (see 1st screenshot below) and the Body has another set of tools (see 2nd screenshot below.)