How to import the http://abc.sharepoint.com/css/custom.css file into the SharePoint Client Web Part?

Hi, 

I am using SharePoint Online. I have created a SharePoint Client Web Part. I want to import the custom.css file located at Url: http://abc.sharepoint.com/css/custom.css into the SharePoint Client Web Part.

Please give code example of how to import the http://abc.sharepoint.com/css/custom.css file into the SharePoint Client Web Part.

Thanks in advance.