OK, you shouldn't modify the theme's files directly; if the theme ever gets updated with new features or with a security patch, then you will lose all of your changes (you don't want to end up like this person). There are two routes you can take: 1) create a child theme (somewhat a bit of overkill for just making CSS changes), or 2) use a plugin like Lazyest Stylesheet.
If you've never installed a plugin before, go to Plugins > Add New in your admin Dashboard. Enter lazyest stylesheet in the search field and click the Search plugins button. Click the Install Now link. After it's done installing, click the Activate link. You will now be able to add your own custom CSS.
From the admin Dashboard, go to Appearance > Lazyest Stylesheet. Copy & paste the CSS rule from my previous message into the big field in the middle and click the blue Save Changes button.