sitemap.xml showing blank page

Since you’ve failed to provide the url to your sitemap, I can’t possibly know what the problem is. The good news is, as long as your sitemap itself is working, Google doesn’t need the stylesheet to see it.
Since I’m lacking any useful information, all I can tell you is that most likely, there’s a plugin or theme conflict, and you can use these troubleshooting steps to determine that: https://semperplugins.com/documentation/how-to-troubleshoot-issues-with-all-in-one-seo-pack/