Yes, you can!
Step 1. Go to Google Tag Manager and add a new tag (create first an account if necessary). Choose Custom HTML tag.
Step 2. Input your CSS in the text box. For example:
font-family: "Comic Sans MS" !important;
Step 3. For the trigger, choose All Pages (should be the default).
Step 4. Give the tag a name and save it. Now submit your Workspace changes.
Step 5. You can now enter the tag in your StoryChief account. Go to Channels and choose William.
Step 6. Go to the Integrations tab. Input the ID of your tag and click on Save.
As you can see below, the font of our blog is now in Comic Sans. Beautiful!