Custom CSS foes

Infos:

• Used Zammad version: 6.2.0-1709021816
• Used Zammad installation type: package
• Operating system: ubuntu 22.04
• Browser + version: Firefox 123.0 (64-bit) and Microsoft Edge for Business 122.0.2365.52 (Official build) (64-bit)

Expected behavior:

• Add custom CSS values to /opt/zammad/app/assets/stylesheets/custom/custom.css like e.g.

  .login {
      background: #ff00ff;
  }
  

  and see the login page with a different background color

Actual behavior:

• Broken CSS with both Firefox and Edge
  

Steps to reproduce the behavior:

• Add custom CSS values to /opt/zammad/app/assets/stylesheets/custom/custom.css like e.g.

  .login {
      background: #ff00ff;
  }
  

• Make zammad the owner with chmod zammad:zammad /opt/zammad/app/assets/stylesheets/custom/custom.css
• Precompile assets with zammad run rake assets:precompile
• Restart service with systemctl restart zammad-web

I looked at all the posts and found only these steps, but somehow I am not able to make it work.
Also reapplying the same steps as here to hide the private/public note as the same issue:

Any hints in the right direction would be appreciated as web design is not my strongest point.

Thanks