@Lauren Helwig - This is not currently possible in either Text or Shape objects.

Here is post about this with a workaround using multiple Shapes:

https://community.miro.com/ask-the-community-45/set-font-attributes-to-specific-words-in-a-shape-3037

I’d recommend adding this as an Idea in the Wish List category by following the guidelines here: Wish List: Everything You Need to Know.

If you do create a Wish List Idea post, I would also recommend that you post a link to it back here, so that future readers of this post can quickly get to and vote for your Idea.

Lastly, don't forget to vote for your own Idea.