Greenhouse Recruiting doesn't include a test environment to preview job board CSS.
However, if your subscription tier includes multiple external job boards, you can create a secondary job board and use this to preview your CSS before applying it to your primary job board. Check out the following article to create a secondary external job board: Add multiple external job boards.
If you're a developer and don't have the necessary Greenhouse Recruiting permissions to create a new job board, reach out to your internal contact to request assistance.