How to draw css class in custom field?

I created a class in my css template (I also tried to add it to the file \ components \ com_osmembership \ assets \ css \ custom.css).
I would draw the class using the field Css Class

in this field change but it seems it does not work.
I doing something wrong?

Sorry, I don't really understand. If you want to use a custom css for the custom field, just edit the field and enter the class you want into CSS Class attribute. It doens't work for you ? Maybe send me the link to the page and explain what field doesn't work so that I can check?

Tuan

I want to change the css style of field type heading.
To do this should be enough just to insert the CSS class that I added to the file custom.css in css class. I found that this may not work because the file
\ components \ com_osmembership \ libraries \ rad \ form \ field \ heading.php the class is already set.
In fact, to get the change of the type field heading I had to edit the file with the name of my class written in custom css.
I hope I was clear.