AdminCP -> Styles & Templates -> Style Manager -> All Style Options (for the Red Fox skin) -> scroll down to Sizes & Dimensions section, change Main Table Width from whatever is there to 100% and save.
Warning: write down the value you find in this setting to start with, because setting this to 100% might screw up your skin, so you want to be able to set it back. If it does mess things up and you still want a full-width look, you will need to find the fluid xml for the skin.