When set to "dark", Vim will try to use colors that look good on a dark background. When set to "light", Vim will try to use colors that look good on a light background. Any other value is illegal. Vim tries to set the default value according to the terminal used. This will not always be correct. Setting this option does not change the background color, it tells Vim what the background color looks like. For changing the background color, see |:hi-normal|.