Is it able to do something "dumber", like just adding a class to the <body> element to then modify what shows up with just CSS? (Tried finding documentation on your site, but seems like it's not a thing).
Yes! Using the editor, you can apply class changes to whatever elements you like (including the <body>). We also expose a JavaScript SDK, so you can further integrate personalization into your application, enabling Mutiny to handle segmentation while you orchestrate any functionality/display changes.