how to theme the read more link using fields mode in views,
You should be making any theme changes in a sub theme, rather than editing the actual theme files. This is so all of your changes are not wiped out when an update is available for corporate clean.

Please read https://www.drupal.org/node/225125 for more information on sub theming.

As for the changes to be made to your stylesheet, open your style.css in an editor such as notepad++ and add the following lines (if align left is what you need)

.more-link {
  text-align:left;
}