If you just want to disable comments on your website, you actually don't need CSS! I've noticed that you've already done so in your website's general settings (Dashboard > Settings > Discussion). Now you need to apply that change manually to the posts and pages that were already published when you changed the general setting. To do so:
- Go to your Dashboard > All Pages page.
- Check that checkbox on the first row, just left of "Title", to select all your pages at once.
- In the Bulk Actions dropdown menu, select "Edit" and then click "Apply".
- You will have several options available for bulk edit. Go to the Comments dropdown menu and select "Do not allow".
- Click the "Update" button on the right side of the page.
You may repeat the same procedure for Dashboard > All Posts as well.