Change Comment

Do you have a link to your website? Just so I could see what we are working with.

You can put this in your custom CSS and it will help a little, but you may have to ultimately adjust your field settings or captcha setting in order to make it not as long:

.comment-form-comment {
  width: 75%;
}
#respond {
  width: 75%;
  margin: auto;
  line-height: 0.5;
}

Here is a screenshot of what it will look like after.
Screenshot

Sorry about that, there is only so much I can do without looking at your back-end. In order to change the background of the form and have control over your text color in the comment box, use the code below and adjust to your liking:

.comment-respond {
  background-color: #D31B2D;
  color: #FFFFFF;
}