`pymdownx.highlight` --- where do I find the "highlight" stylesheet used by syntax highlighting on fenced blocks?

I've been trying to write a Flask app which automatically converts Markdown into an HTML that will get served as the response.

Consider the following toy snippet:

import markdown
md = markdown.Markdown(extensions=['pymdownx.superfences'])

s = '''
```py3
import hello_world
\```  # ignore the backslash here
'''.strip()

print(md.convert(s))

And it produces the following output:

<div class="highlight"><pre><span></span><code><span class="kn">import</span> <span class="nn">hello_world</span>
</code></pre></div>

Notice the class="highlight" there in the div. My produced HTML has no syntax highlighting, obviously---because I don't have a stylesheet that defines that "highlight" class. But for the life of me, I can't figure out what I'm supposed to do with pymdownx in order to find that stylesheet with highlight. Is there a property or function I need to access? Is there something separate that I need to include in the CSS on my own?

I'm pretty lost, so any help would be appreciated.