This is my css for reading ebooks in Calibre.

calibre.css

/** 
  * Font: EB Garamond <http://www.georgduffner.at/ebgaramond/>
  * and use of 2 columns
  */
    

html {
  line-height: 1.5em;
  text-align: justify;
  font-family: "EB Garamond" !important;
  text-indent: 2em;

  /* iPad
  background-color: #f5efdc;
  color: #000;
  */

  /* Kindle
  background-color: #fbf0d7;
  color: #453722;
  */

  /* Readable
  background-color: rgb(247, 247, 247);
  color: rgb(43, 55, 61);
  */

  /* iPad + Kindle */
  background-color: #f5efdc;
  color: #453722;
}