Calibre's viewer is quite capable, but as many have pointed out, staring at a computer monitor for any length of time begins to strain the eyes.
I've modified the css from
Ekaser's post, to create a dark charcoal gray background with barely gray text. Resulting in distinct, but not too extreme, contrast to avoid a "glare" effect.
The margins are very wide (350px) to create a column of justified text within the center of the screen (intended for 20"+ monitors, otherwise margins will need editing downward). Note that the viewer is used full window. (Not full screen - which removes the option to see the taskbar & system tray.)
I've manually increased font size in the viewer, not in the CSS. That is the only un-accounted for change in the viewer snapshot. (I imagine font size can be set via CSS, but sometimes I vary the size.)
Code:
body {
color: rgb(235,235,235);
background-color:rgb(20,20,20);
text-align:justify;
line-spacing:1.8;
margin-top:0px;
margin-bottom:4px;
margin-right:350px !important;
margin-left:350px !important;
text-indent:2em;
}
h1, h2, h3, h4, h5, h6 {
color: rgb(235,235,235);
text-align:center;
font-style:italic;
font-weight:bold;
}
This results in a a fairly dark, monochrome color palette which prevents desktop elements from distracting you while reading - while leaving them available if you quickly need to access them.
If you would like darker text, change the topmost "color: rgb(235,235,235);" line to a
lower number. Keeping all three sets of numbers the same will result in a true gray.