No margin on article's header

Hello,

I've added a header new element in which I display the title of the article.
I applied a custom class + margin-bottom on the properties of this element.

In case the margins applied which the properties in the element weren't enough, I added to my custom-class :
`.art-header {
margin-bottom: 50px !important;
}'

But it's not working (and it's the weirdest thing) : It's working for me, but not for others computers, nor when I'm on private navigation on the same browser.

The custom class isn't working and I don't know why.

Thanks !