Hi Ramalingaiah,
When you say a custom header, do you mean a custom <head/> tag? or do you just mean the header as in a <div> that you put things in (site name, logo, navigation, etc).
Adding a css class is no different than any other application. All you have to do is add the <link> tag in the <head> section at the top of the file. If the References to external sites don't require anything special. If you are looking to add custom css (files) to your theme, then you should be using the _scss (Saas) format. The _custom.scss has a reference to the _main.scss .. and in there you can add to the list the file that you want to add. For example, you would add a file called _my-custom.scss, with your definitions. Then in the _main.scss, under the @import you would add "my-custom"
If I have understood your question, that should be all you need.
