CSS Studio leverages your existing agent - so it assumes you've already done this part. Where this comes in is when you want to design not just using chat (although it does also support chat). Drawing new elements, visual style controls, inline content editing, animation timeline editor and preview. Then once you're done you can send those changes to your agent with a click (or turn on auto-apply)
It does have some similarities with Figma's Code Layers in that you can draw a new element on the page, click chat, and tell the agent to generate x inside.