What you can do with CSS is change the appearance of something already on the page, and there is also a way to add plain text using pseudo elements, but you can't change out just one word of a string such as "comment" in "1 comment" using CSS only. If you really wanted to get full control to change that wording, you would need to switch to WordPress.org and actually do some custom PHP programming or find a plugin that does what you want.
That said, here are some examples of adding text using pseudo elements that you can play around with.
I see you replaced the "leave-reply" link text using this custom CSS:
.leave-reply {
visibility: hidden;
}
.leave-reply:before {
visibility: visible;
content:"What'd you think? Leave a comment.";
}
Another way to do that would be to just add the first part like this:
.leave-reply:before {
content: "What'd you think? ";
}
And if you wanted to do the same thing when some comments have already been left, you would also add this:
.comments-link:before {
content: "What'd you think? ";
}
Or if you wanted to add "Leave your own." after the links that show the number of comment that have been left, you could add this:
.comments-link:after {
content: " Leave your own.";
}