When you open a new site for editing you can easily set the site's background using the Site Presets window, which can be found on the top left corner under Settings menu.
The Site Presets window is divided into 3 sections: Size & Position, Page Setup and Color Style.
Size & Position.
This section refers to the site's width and alignment. When changing site's width, make sure to use standard screen resolutions like 1024, 1280 and etc.
Page Setup Menu.
Background refers to the fill of the entire browser, what is behind and beyond the site's size.
Fill refers to the site's content area.
Stroke and shadow refer to the outer edge of the site's content area.
Note: In both background and fill settings you can set up a coloured background or upload a background image
You can save your colour swatches for the site easily and they will be saved, enabling you to easily select them for any element you are designing. If you'll choose to change one of the colours, all elements with this selected colour will change accordingly.