Field styles are a complicated mechanism. The system has to check if a value in a column equals the value defined in the style record. So many things can go wrong here, right? Well…actually, yes. It’s so complicated that ServiceNow has given up fixing it, as per some Knowledge Base articles.
Here’s one example I recently encountered when trying to apply styles to a Boolean field. The requirements were to apply the style to a List and a report, specifically a Calendar report.