We had this question earlier in a reddit submission so copying and pasting the answer here (so that you don't think I'm being insincere!) - We fully support styledComponents, or any valid React for that matter. You can even use utility CSS classes in code. The design tool can edit some of these today already (e.g. you can pass in additional styles to a component instance). Better support for creating and managing styledComponents / Emotion is high on our roadmap so watch this space!
I had the same thought about styled components. In the meantime I'll just use emacs to convert the default style objects to styled components when I'm done using Utopia.
I imagine I will use Utopia and then tailor the code afterwards with a code-editor to add my own idiosyncrasies. Still pretty darn efficient to design UIs this way honestly. I'm pretty stoked!