I think that implementing a choice to change the default color of a page in a story game would be beneficial to the readers. It would add a feel to it. For example, if you were playing a horror story game, wouldn't the reader feel more immersed if they were staring at a dark screen, instead of a bright white one?
And of course don't allow the authors to implement crazy colors, and maybe if the readers didn't want the different background color, maybe give them an option to disable it?
But I know that it is mostly about the actual words that create the atmosphere, but to create that true environment (or at least take a step towards it), I believe that it truly would be a great implementation into the site.