I'm not familiar with doing this, but I would assume you could add it to Settings>SEO>Header. You would need to make sure to include the open and closing tags for the style code, so <style> at the beginning of the code and </style> at the end of the code (assuming those are the correct open/close tags). Technically, you could also add it under Theme>CSS/HTML but I would recommend trying this first option I provided since editing the theme area will flag your site as "custom". It can be difficult to troubleshoot a custom site if you run into an issue or need to contact support, add it's much easier to remove code from Settings>SEO>Header to verify the third party code is not causing any potential issues.