Note: Greenhouse's new job board customization contains built-in styling options that remove the need for custom CSS. Uploading a custom CSS file is only available to organizations still on the legacy job board, and will eventually be removed. Read more about the new job board configuration options here.
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.