How can I make custom changes to Semantic UI CSS with Meteor?

I built my entire front end of my app with Semantic UI + React and now when I just have a landing page left to do, it seems like I can’t proceed at all.

I’m trying to add masthead classes to my app for the landing page, but there is nothing I can edit (that I know of) that would let me make these changes without the CSS resetting to default when my app saves.

Meteor always made styling a read pain in the ass, but I figure that there is some way that maybe someone has found to make these changes. Any input is much appreciated.