To get closer to the Kindle-for-PC reading experience (esp. the less-distracting dark screen on either side of the text), I've been using this stylesheet, with the Calibre viewer maximized (on a 1600x900 resolution monitor):
body {color:RGB(0,0,0) !important;
background-color:RGB(253,253,253) !important;
text-align:justify !important;
line-height:1.2 !important;
margin-top:0px !important;
margin-bottom:4px !important;
margin-right:450px !important;
margin-left:450px !important;
text-indent:5em !important;
background-image:url("http://img33.imageshack.us/img33/2545/calibrebackground.png") !important;
background-position:50% !important;}
p,div {margin-bottom:8px !important;margin-top:8px !important;}
blockquote {padding-right:35px !important;margin-bottom:8px !important;}
h1, h2, h3, h4, h5, h6
{color:black !important;
text-align:center !important;
font-style:italic !important;
font-weight:bold !important;}
The calibrebackground.png image is just a 1600 pixel by 5 pixel graphic, with the dark-gray color sampled from the Kindle PC reader. If you're using a different screen resolution, you'd need to play around with the body margin-left and margin-right properties, and probably modify the .png too.
