Applying Css on Dynamics CRM Entity Form

Hi All, How can we set Css properties on Dynamics 365 Entity Form. I have read some articles and created CSS and load it through Java script. but it has not shown any change in the design. Any body help me? Following is my CSS and Javascript code.

CSS Code

h2

{
background-color: black;
color:white;
font-size:30px;
}

JS Code to Load this CSS

function LoadCss()
{
debugger;
var path = ""/WebResources/nfs_designiframe.css"";
var head = window.parent.document.getElementsByTagName('head')[0];
var link = window.parent.document.createElement('link');
link.rel = 'stylesheet';
link.type = 'text/css';
link.href = path;
link.media = 'all';
window.parent.document.head.appendChild(link);
}

Firstly i have taken a simple H2 tag as selector in Css just to check if it is working. It represents the headers of Tab like "General".

It is not working, I don't know but may be the path format is incorrect. Any one know please help me?