Custom CSS not working

I want to create a subhead using H6 and I want to reduce the space (margin) beneath it, here is the custom CSS I used:

h6 {
color: #FF0000;
font-weight: 600;
margin: 10px 0 0px;
}

But it’s not changing on the web page, there’s still a huge space beneath the headline.

Site is not live yet.

Hi,

Please post a link to your website, to the page where you have that heading, so I can check.

Kind Regards,
Csaba