Changing header padding

Hi there!
I’m trying to decrease the padding of the header in the custom stack in card, but I’m not being successful in my task!
Can you provide me some guidance?
I was able to change css for other cards but not for this one!

Thanks a lot for your help!!

            $: |
              h1.card-header {
                font-size: 16px;
                padding-top: 1px;
                padding-bottom: 18px; 

I think this is what you need