Which social media library does this theme use?

Genericons (which Minnow uses for its social menu) doesn’t include a Medium icon, but you could get the Tumblr icon working with custom CSS – I’d be glad to help.

Looks like you marked this thread as resolved, though, so are you all set?

Thank you for your offer, but I found a few tutorials on how to use Font Awesome (which includes Medium icons and Tumblr icons (for custom domains)) with WordPress.

Excellent, glad you got it sorted!

I would be interested to know how to get a Tumblr icon working, and also if there’s a way to get a “phone” icon for a tel: link (for phone numbers).

Actually I figured this out. It’s a little tricky because the old Genericons site is gone, replaced by Genericons Neue, which is not the same thing. But I found the needed codes on Github.

Here’s the custom CSS to add a few new icon types.

.social-links ul a[href*='tel:']:before {
    content: '\f437';
}
.social-links ul a[href*='tumblr']:before {
    content: '\f214';
}
.social-links ul a[href*='goodreads']:before {
    content: '\f444';
}