Easy. After opening a new file, go to the Others folder in the Atmosphere dialogue and choose either White Back or Black Back. This will give you either a totally white or black sky. If you want some other colour, open the Atmosphere Editor, go to the Sky tab and double-click on the Sky Colour Map box. In the browser, choose the colour you would like (either plain or a colour gradient if preferred). OK everything then select Ground in the object hierarchy tree and press delete. You now have a plain (or graduated) background in the colour of your choosing.