Greenhouse Recruiting does not include a test environment to preview job board CSS.

However, if your subscription 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. Use the steps in the following article to create a secondary external job board: Add Multiple External Job Boards

If you are a developer and do not have the necessary Greenhouse Recruiting permissions which enable you to create a new job board, reach out to your internal contact to request assistance.