Edit the eBook in the Calibre editor. Remove all text-align: left; from the CSS. In the body CSS, add in text-align: justify; See the body CSS style that you should add in or replace body with.
Code:
body {
widows: 1;
orphans: 1;
margin-top: 0;
margin-right: 0;
margin-bottom: 0;
margin-left: 0;
text-align: justify;
}