Modify margins for h2 elements

In posts, it seems that the theme sets the following margins on <h2> elements:
margin-top: 10px;
margin-botton: 24px;

How can I change that? Using Simple Custom CSS plugin I added:
h2 {
margin-bottom: 10px;
margin-top: 24px;
}

But this did not seem to have any effect

Hi,

For blog listings page you can use custom css below (this is default value):

body.blog h2.title-post {
padding-bottom: 10px;
}

For single post you can use custom css below (this is default value):

body.single h1.title-post {
padding-bottom: 10px;
}

Or, please describe us what you want to achieve, and we will try to help.

Best Regards