In your wp-content/themes/simplecss/ folder, open up the style.css file and scroll to line 580 (approx). You will find the code for the commentlist.

Replace the following:

.commentlist {
 padding: 0;
 text-align: justify;
 }
.commentlist li {
 margin: 15px 0 3px;
 padding: 5px 10px 3px;
 list-style: none;
 }
.commentlist p {
 margin: 10px 5px 10px 0;
 }

with this:

#commentlist {
 padding: 0;
 text-align: justify;
 list-style-type: none;
 }
#commentlist ol {
 padding: 0 0 0 35px;
 margin: 0;
 list-style-type: none;
 }
#commentlist li {
 margin: 15px 0 3px;
 padding: 5px 10px 3px;
 list-style: none;
 }
#commentlist p {
 margin: 10px 5px 10px 0;
 }

Save or update your file. 

Complete