Hi @Piyush Meshram ,
A CHM file can be thought of as a zipped (compiled) web site. The content pane of the help viewer on the right displays the content like a browser window. If you want to see the same formatting by .css as on your web site you'll have to integrate these .css files into the CHM file.
On the Internet you can find many tools to create CHM files.
This forum discuss the same and might be helpful https://social.msdn.microsoft.com/Forums/expression/en-US/2062159b-d906-4917-830b-335e16545b1b/how-to-add-a-chm-help-file-to-my-website?forum=web
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