It is a bit tricky! You would add the code to the CSS editor in the Appearance > Customize > CSS panel in your blog dashboard.
Now, keep in mind I used the Pinterest link you already have on your blog as an example, so it's a starting point but not finished yet. If you want to try it out, I was thinking you could add your link to any one of the currently unused link fields in Appearance > Customize > Theme Options in the Singl theme and then I could see what I can come up with to try to adjust that one. If you're willing to give it ago, go ahead and add your link and then I'll help you with a CSS example specific to what you add.
This request looks like a good challenge. :)