Sorry, I could’ve been much clearer. 
For that you can add it to the css of your theme by going to /admin/customize/themes, selecting a theme, and clicking on the ‘Edit CSS/HTML’ button. Then copy the code above and paste it into the common tab (and save).
Though it’s better to add it to a theme component if you can, as that will stop it getting overwritten in any updates (and you can attach it to multiple themes). It’s very similar to the above, but with a few extra button presses. You can find out a bit more about that in: