Custom CSS can be applied for any of the selected columns, listed in the left-hand section of the Fields tab. Click on the gear icon beside any column name & it will reveal a couple of free text fields where custom CSS can be added.
Header styles refer to the styling applied to the table header for that specific column whereas Entry styles CSS attributes are applied to all the data points generated within that column.
If the JQL section data is grouped by a specific column, it is possible to apply custom CSS to that data as well. Just click on the Gear icon beside Group by option. It shows the custom CSS options.