Half the web runs on WordPress. WordPress sitemap files contain stylesheets.
So, I don't believe that stylesheets on their own are a problem for Google.
I have several sites with stylesheets in sitemap files - and all worked OK last time I looked.
Stylesheets exist to convert XML to HTML for browsers, and Google wouldn't want to convert them.
So it is not obvious to me what the issue is with your site.
Maybe there is something else co-incidental?
I can confirm the stylesheets are not the problem. Removing the lines as suggested doesn't solve my problem.
After adding the sitemap to Googles search console, it immediately says: "sitemap can't be read"
EDIT: Google seemst to try to get the sitemap though
ravenzwaaij.info anon-0-0-0-253.ip6.invalid - - [28/Mar/2023:17:48:09 +0200] "GET /admin HTTP/1.1" 200 4486 "https://www.ravenzwaaij.info/tree/ravenzwaaij/my-page" "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/111.0.0.0 Safari/537.36 Edg/111.0.1661.54"
ravenzwaaij.info anon-0-0-0-253.ip6.invalid - - [28/Mar/2023:17:48:16 +0200] "GET /module/sitemap/Admin HTTP/1.1" 200 2010 "https://www.ravenzwaaij.info/admin" "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/111.0.0.0 Safari/537.36 Edg/111.0.1661.54"
ravenzwaaij.info anon-0-0-0-253.ip6.invalid - - [28/Mar/2023:17:48:50 +0200] "GET /sitemap.xml HTTP/1.1" 200 272 "https://search.google.com/" "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/111.0.0.0 Safari/537.36 Edg/111.0.1661.54"
ravenzwaaij.info anon-0-0-0-253.ip6.invalid - - [28/Mar/2023:17:48:51 +0200] "GET /sitemap.xsl HTTP/1.1" 200 824 "https://www.ravenzwaaij.info/sitemap.xml" "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/111.0.0.0 Safari/537.36 Edg/111.0.1661.54"
ravenzwaaij.info anon-144-187-87-33.ip6.invalid - - [28/Mar/2023:17:54:13 +0200] "GET /tree/ravenzwaaij HTTP/1.1" 200 5631 "-" "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/111.0.0.0 Safari/537.36 Edg/111.0.1661.54"
ravenzwaaij.info anon-144-187-87-33.ip6.invalid - - [28/Mar/2023:17:54:16 +0200] "GET /tree/ravenzwaaij/tree-page-block?block_id=63 HTTP/1.1" 200 1894 "https://www.ravenzwaaij.info/tree/ravenzwaaij" "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/111.0.0.0 Safari/537.36 Edg/111.0.1661.54"
ravenzwaaij.info anon-144-187-87-33.ip6.invalid - - [28/Mar/2023:17:54:16 +0200] "GET /tree/ravenzwaaij/tree-page-block?block_id=41 HTTP/1.1" 200 1683 "https://www.ravenzwaaij.info/tree/ravenzwaaij" "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/111.0.0.0 Safari/537.36 Edg/111.0.1661.54"
ravenzwaaij.info anon-144-187-87-33.ip6.invalid - - [28/Mar/2023:17:54:40 +0200] "GET /sitemap.xml HTTP/1.1" 200 237 "-" "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/111.0.0.0 Safari/537.36 Edg/111.0.1661.54"