As per following support articles if you want to apply a field style to both form and list then value field of style has to be left blank but issue with that is, it will apply style to list which is not expected, for example with below field style output on form and list would look like below

https://support.servicenow.com/kb?id=kb\_article\_view&sysparm\_article=KB0584393

https://support.servicenow.com/kb?id=kb\_article\_view&sysparm\_article=KB0564026

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