where is "comments.php" ??

Try using Firefox with the Firebug add-on for this kind of CSS work.
https://getfirebug.com/

Can you please tell the theme you are using?

Normally comments.php will be in the home folder of the theme-directory.

I’m using the headway theme

As you are using a commercial theme, you need to seek support from the theme’s vendors.

ah – ok! cheers

(i might also try the firebug too – will let you know if it works)