Steps to reproduce
Login as System
Open CSS Style window and create a CSS style with a background-color: red;.
Open Window, Tab & Field Window and open the Request window
Navigate to tab and field and set the newly created style in Label Style for Document No
Check the Heading only flag.
Logout and log back in as Garden Admin
Open the Request window
The DocumentNo label should have a red background
The DocumentNo label is displayed with the normal background-colour
I configured the above example in demo.globalqss.com you can check it by opening the request window