Can't change font-size in the Header of Accordion -Visual Composer

Hi ,
i would like the font-size in the Header of Accordion (Visual Composer ) but when i input the in the CSS Tool the code to change it there is not modify .

In the accordion>header inside the field ‘Extra class name’ i input “dudu” then i open the CSS Editor writing this code : .dudu {font-size : 10px;} when i save nothing happen.

Do you have any suggestion to work out this step?

Thanks

Jay