Adding a new H1 style in RoboHelp 2017

Hi, I need to create another Heading 1, which is identical to the existing Heading 1 style, but with a different background colour. I assumed this would be straight forward but I can't get the style to display correctly in the Responsive HTML 5 output. I have tried copying the existing Heading 1 style in the Styles editor in RoboHelp, and have also tried changing the formatting of the existing Heading 6 style, but when I create the output, the style is not applied correctly. I think I must be making a simple mistake... I have added the existing Heading 1 style below. Any advice would be appreciated.

h1 {

    font-family:Arial, sans-serif;

    font-size:1.2em;

    font-weight:bold;

    min-height:1.2em;

    color:#ffffff;

    background-color:slategray;

    padding:0.5em 0em 0.5em 0.65em;

    margin:0.65em 0em 0.6em 0em;

}