Customize "Post A Comment" Page

I have enabled comments for my site. I have added a post and I can write comments. When someone new writes a comment they are sent to a "Post A Comment" page where they are asked to enter a name, email and website URL. My audience is elderly people and I wish to minimize confusion by removing the website url field. It is not required to submit the post and it is sure to confuse people. 

I was trying to set display:none for the input field or the surrounding div tag but I am not having any luck trying to modify that page. Even something as simple as  trying to set the background-color of a div on the page is not working. 

Here is the CSS I added to Custom CSS under WEBSITE TOOLS

input[placeholder="Website URL"] {
  display: none;
}

Ok, that code works. I had an unclosed comment before the line of code that was causing the line to become commented out but I didn't notice it because the text formatting in the CSS editor was not highlighting the line as a comment.