I've got a light colored wordpress theme. I've designed a Dark version of the theme by looking into making a brand new version from the css.

I would like site visitors to have the ability to vary from the sunshine towards the dark version, basically just altering which CSS file they're using.

Is easy to do?

Yes, you could do.

You need to check what option(light or dark) they selected and can include style file accordingly.

if($theme_selected == 'light')
{
  echo '<link rel="stylesheet" type="text/css" 
        href="http://mydomain.com/theme_url/light.css" />';
}
else
{
   echo '<link rel="stylesheet" type="text/css" 
          href="http://mydomain.com/theme_url/dark.css" />';
}

Take a look in the following links to obtain began having a theme options page in Wordpress. You'll have the ability to let someone choose a "gray" theme, or perhaps a "light" theme, or anything you set. Whatever is occur the admin will function as the loaded stylesheet.

You need to consider the "color plan" thing about this tutorial:

http://net.tutsplus.com/tutorials/wordpress/how-to-create-a-better-wordpress-options-panel/