Removing white lines showing bellow each column of my table

Look at default.css – line 196

.commentlist li.comment article[id*="comment-"] table td, .post_content table td {
 border-bottom: 1px solid #D0D0D0;
 padding: 5px;
}

Dear Andrew,
Thank you so much for such quick response but please guide me where to make change. I have bit different code in CSS which is as follow:
.commentlist li.comment {
background:#fafafa;
border:1px solid #cfd3d6;
border-radius:7px;
box-shadow: rgba(0, 0, 0, 0.5) 1px 1px 0p
Please tell me where to make change and what change should I made?

Which file are you looking at?

Andrew, I am looking at style.css file, In this theme there are three css files named:
Visual editor stylesheet.css
Rtl style sheet.css
stylesheet.css
There is no default.css file

Default CSS looks like it’s coming from a plugin, https://10-for-10.com/wp-content/themes/online-marketer/library/css/default.css .

Have you a Child Theme or Custom CSS/ Custom Style section in the dashboard?

Its not a child theme.

Is it a theme you made yourself, or which theme?

I can also see the above information which you shared with me when I inspect the table with firebug. In fact I am making this page for someone, and I don’t have access to c panel or ftp account. Can I access this default.css file through wordpress?

If you want, you can try to override this style on the stylesheets you can access, by using !important prepended on the CSS value.
E.g

.commentlist li.comment article[id*="comment-"] table td, .post_content table td {
 border-bottom: 0 !important;
}

Here’s a resource on CSS !important
https://webdesign.about.com/od/css/f/blcssfaqimportn.htm

Andrew, I recently downloaded this theme and found default.css file there. It means that default.css file exist in the theme then why its not showing in the list of files when I try to edit the theme.

As you’ve downloaded this theme, you ought to use a Child Theme or Custom CSS Manager, which allow you to make modifications without editing original theme files, otherwise modifications will erase once the theme updates.