When a user isn't logged in, there is a caption above the comments that says, 'Only registered users can write comments!'

I'd like to change the size and color of this caption but I can't find where I do this. From firebug, I see that this is located in the file:

/components/com_comment/joscomment/templates/magazineJoomla/js/custom.js" type="text/javascript">

But when I enter that file to edit, I cannot find where I can make these changes.